Let's continue the conversation about the least common multiple, which we started in the section “LCM - least common multiple, definition, examples.” In this topic, we will look at ways to find the LCM for three or more numbers, and we will look at the question of how to find the LCM of a negative number.
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Calculating Least Common Multiple (LCM) via GCD
We have already established the relationship between the least common multiple and the greatest common divisor. Now let's learn how to determine the LCM through GCD. First, let's figure out how to do this for positive numbers.
Definition 1
You can find the least common multiple through the greatest common divisor using the formula LCM (a, b) = a · b: GCD (a, b).
Example 1
You need to find the LCM of the numbers 126 and 70.
Solution
Let's take a = 126, b = 70. Let's substitute the values into the formula for calculating the least common multiple through the greatest common divisor LCM (a, b) = a · b: GCD (a, b) .
Finds the gcd of numbers 70 and 126. For this we need the Euclidean algorithm: 126 = 70 1 + 56, 70 = 56 1 + 14, 56 = 14 4, therefore GCD (126 , 70) = 14 .
Let's calculate the LCM: LCD (126, 70) = 126 70: GCD (126, 70) = 126 70: 14 = 630.
Answer: LCM(126, 70) = 630.
Example 2
Find the number 68 and 34.
Solution
GCD in in this case This is not difficult, since 68 is divisible by 34. Let's calculate the least common multiple using the formula: LCM (68, 34) = 68 34: GCD (68, 34) = 68 34: 34 = 68.
Answer: LCM(68, 34) = 68.
In this example, we used the rule for finding the least common multiple of positive integers a and b: if the first number is divisible by the second, the LCM of those numbers will be equal to the first number.
Finding LCM by factoring numbers
Now let's look at the method of finding the LCM, which is based on factoring numbers into prime factors.
Definition 2
To find the least common multiple, we need to perform a number of simple steps:
- we compose the product of all prime factors of the numbers for which we need to find the LCM;
- we exclude all prime factors from their resulting products;
- the product obtained after eliminating the common prime factors will be equal to the LCM of the given numbers.
This method of finding the least common multiple is based on the equality LCM (a, b) = a · b: GCD (a, b). If you look at the formula, it will become clear: the product of the numbers a and b is equal to the product of all the factors that participate in the decomposition of these two numbers. In this case, the gcd of two numbers is equal to the product of all prime factors that are simultaneously present in the factorizations of the given two numbers.
Example 3
We have two numbers 75 and 210. We can factor them as follows: 75 = 3 5 5 And 210 = 2 3 5 7. If you compose the product of all the factors of the two original numbers, you get: 2 3 3 5 5 5 7.
If we exclude the factors common to both numbers 3 and 5, we get a product of the following form: 2 3 5 5 7 = 1050. This product will be our LCM for the numbers 75 and 210.
Example 4
Find the LCM of numbers 441 And 700 , factoring both numbers into prime factors.
Solution
Let's find all the prime factors of the numbers given in the condition:
441 147 49 7 1 3 3 7 7
700 350 175 35 7 1 2 2 5 5 7
We get two chains of numbers: 441 = 3 3 7 7 and 700 = 2 2 5 5 7.
The product of all factors that participated in the decomposition of these numbers will have the form: 2 2 3 3 5 5 7 7 7. Let's find common factors. This is the number 7. Let's exclude him from total product: 2 2 3 3 5 5 7 7. It turns out that NOC (441, 700) = 2 2 3 3 5 5 7 7 = 44 100.
Answer: LOC(441, 700) = 44,100.
Let us give another formulation of the method for finding the LCM by decomposing numbers into prime factors.
Definition 3
Previously, we excluded from the total number of factors common to both numbers. Now we will do it differently:
- Let's factor both numbers into prime factors:
- add to the product of the prime factors of the first number the missing factors of the second number;
- we obtain the product, which will be the desired LCM of two numbers.
Example 5
Let's return to the numbers 75 and 210, for which we already looked for the LCM in one of the previous examples. Let's break them down into simple factors: 75 = 3 5 5 And 210 = 2 3 5 7. To the product of factors 3, 5 and 5 numbers 75 add the missing factors 2 And 7 numbers 210. We get: 2 · 3 · 5 · 5 · 7 . This is the LCM of the numbers 75 and 210.
Example 6
It is necessary to calculate the LCM of the numbers 84 and 648.
Solution
Let's factor the numbers from the condition into simple factors: 84 = 2 2 3 7 And 648 = 2 2 2 3 3 3 3. Let's add to the product the factors 2, 2, 3 and 7
numbers 84 missing factors 2, 3, 3 and
3
numbers 648. We get the product 2 2 2 3 3 3 3 7 = 4536. This is the least common multiple of 84 and 648.
Answer: LCM(84, 648) = 4,536.
Finding the LCM of three or more numbers
Regardless of how many numbers we are dealing with, the algorithm of our actions will always be the same: we will sequentially find the LCM of two numbers. There is a theorem for this case.
Theorem 1
Let's assume we have integers a 1 , a 2 , … , a k. NOC m k these numbers are found by sequentially calculating m 2 = LCM (a 1, a 2), m 3 = LCM (m 2, a 3), ..., m k = LCM (m k − 1, a k).
Now let's look at how the theorem can be applied to solve specific problems.
Example 7
You need to calculate the least common multiple of four numbers 140, 9, 54 and 250 .
Solution
Let us introduce the notation: a 1 = 140, a 2 = 9, a 3 = 54, a 4 = 250.
Let's start by calculating m 2 = LCM (a 1 , a 2) = LCM (140, 9). Let's apply the Euclidean algorithm to calculate the GCD of the numbers 140 and 9: 140 = 9 15 + 5, 9 = 5 1 + 4, 5 = 4 1 + 1, 4 = 1 4. We get: GCD (140, 9) = 1, GCD (140, 9) = 140 · 9: GCD (140, 9) = 140 · 9: 1 = 1,260. Therefore, m 2 = 1,260.
Now let’s calculate using the same algorithm m 3 = LCM (m 2 , a 3) = LCM (1 260, 54). During the calculations we obtain m 3 = 3 780.
All we have to do is calculate m 4 = LCM (m 3 , a 4) = LCM (3 780, 250). We follow the same algorithm. We get m 4 = 94 500.
The LCM of the four numbers from the example condition is 94500.
Answer: NOC (140, 9, 54, 250) = 94,500.
As you can see, the calculations are simple, but quite labor-intensive. To save time, you can go another way.
Definition 4
We offer you the following algorithm of actions:
- we decompose all numbers into prime factors;
- to the product of the factors of the first number we add the missing factors from the product of the second number;
- to the product obtained at the previous stage we add the missing factors of the third number, etc.;
- the resulting product will be the least common multiple of all numbers from the condition.
Example 8
You need to find the LCM of five numbers 84, 6, 48, 7, 143.
Solution
Let's factor all five numbers into prime factors: 84 = 2 2 3 7, 6 = 2 3, 48 = 2 2 2 2 3, 7, 143 = 11 13. Prime numbers, which is the number 7, cannot be factored into prime factors. Such numbers coincide with their decomposition into prime factors.
Now let's take the product of the prime factors 2, 2, 3 and 7 of the number 84 and add to them the missing factors of the second number. We decomposed the number 6 into 2 and 3. These factors are already in the product of the first number. Therefore, we omit them.
We continue to add the missing multipliers. Let's move on to the number 48, from the product of whose prime factors we take 2 and 2. Then we add the prime factor of 7 from the fourth number and the factors of 11 and 13 of the fifth. We get: 2 2 2 2 3 7 11 13 = 48,048. This is the least common multiple of the original five numbers.
Answer: LCM(84, 6, 48, 7, 143) = 48,048.
Finding the least common multiple of negative numbers
In order to find the least common multiple of negative numbers, these numbers must first be replaced by numbers with the opposite sign, and then the calculations must be carried out using the above algorithms.
Example 9
LCM (54, − 34) = LCM (54, 34) and LCM (− 622, − 46, − 54, − 888) = LCM (622, 46, 54, 888).
Such actions are permissible due to the fact that if we accept that a And − a– opposite numbers,
then the set of multiples of a number a matches the set of multiples of a number − a.
Example 10
It is necessary to calculate the LCM of negative numbers − 145 And − 45 .
Solution
Let's replace the numbers − 145 And − 45 to their opposite numbers 145 And 45 . Now, using the algorithm, we calculate the LCM (145, 45) = 145 · 45: GCD (145, 45) = 145 · 45: 5 = 1,305, having previously determined the GCD using the Euclidean algorithm.
We get that the LCM of the numbers is − 145 and − 45 equals 1 305 .
Answer: LCM (− 145, − 45) = 1,305.
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Greatest common divisor and least common multiple are key arithmetic concepts that allow you to operate effortlessly ordinary fractions. LCM and are most often used to find the common denominator of several fractions.
Basic Concepts
The divisor of an integer X is another integer Y by which X is divided without leaving a remainder. For example, the divisor of 4 is 2, and 36 is 4, 6, 9. A multiple of an integer X is a number Y that is divisible by X without a remainder. For example, 3 is a multiple of 15, and 6 is a multiple of 12.
For any pair of numbers we can find their common divisors and multiples. For example, for 6 and 9, the common multiple is 18, and the common divisor is 3. Obviously, pairs can have several divisors and multiples, so the calculations use the largest divisor GCD and the smallest multiple LCM.
The least divisor is meaningless, since for any number it is always one. The greatest multiple is also meaningless, since the sequence of multiples goes to infinity.
Finding gcd
There are many methods for finding the greatest common divisor, the most famous of which are:
- sequential enumeration of divisors, selection of common ones for a pair and search for the largest of them;
- decomposition of numbers into indivisible factors;
- Euclidean algorithm;
- binary algorithm.
Today at educational institutions The most popular are the methods of prime factorization and the Euclidean algorithm. The latter, in turn, is used when solving Diophantine equations: searching for GCD is required to check the equation for the possibility of resolution in integers.
Finding the NOC
The least common multiple is also determined by sequential enumeration or decomposition into indivisible factors. In addition, it is easy to find the LCM if the greatest divisor has already been determined. For numbers X and Y, the LCM and GCD are related by the following relationship:
LCD(X,Y) = X × Y / GCD(X,Y).
For example, if GCM(15,18) = 3, then LCM(15,18) = 15 × 18 / 3 = 90. The most obvious example of using LCM is to find the common denominator, which is the least common multiple of given fractions.
Coprime numbers
If a pair of numbers has no common divisors, then such a pair is called coprime. The gcd for such pairs is always equal to one, and based on the connection between divisors and multiples, the gcd for coprime pairs is equal to their product. For example, the numbers 25 and 28 are relatively prime, because they have no common divisors, and LCM(25, 28) = 700, which corresponds to their product. Any two indivisible numbers will always be relatively prime.
Common divisor and multiple calculator
Using our calculator you can calculate GCD and LCM for an arbitrary number of numbers to choose from. Tasks on calculating common divisors and multiples are found in 5th and 6th grade arithmetic, but GCD and LCM are key concepts in mathematics and are used in number theory, planimetry and communicative algebra.
Real life examples
Common denominator of fractions
Least common multiple is used when finding the common denominator of multiple fractions. Let in arithmetic problem you need to sum 5 fractions:
1/8 + 1/9 + 1/12 + 1/15 + 1/18.
To add fractions, the expression must be reduced to common denominator, which reduces to the problem of finding the LCM. To do this, select 5 numbers in the calculator and enter the values of the denominators in the appropriate cells. The program will calculate the LCM (8, 9, 12, 15, 18) = 360. Now you need to calculate additional factors for each fraction, which are defined as the ratio of the LCM to the denominator. So the additional multipliers would look like:
- 360/8 = 45
- 360/9 = 40
- 360/12 = 30
- 360/15 = 24
- 360/18 = 20.
After this, we multiply all the fractions by the corresponding additional factor and get:
45/360 + 40/360 + 30/360 + 24/360 + 20/360.
We can easily sum such fractions and get the result as 159/360. We reduce the fraction by 3 and see the final answer - 53/120.
Solving linear Diophantine equations
Linear Diophantine equations are expressions of the form ax + by = d. If the ratio d / gcd(a, b) is an integer, then the equation is solvable in integers. Let's check a couple of equations to see if they have an integer solution. First, let's check the equation 150x + 8y = 37. Using a calculator, we find GCD (150.8) = 2. Divide 37/2 = 18.5. The number is not an integer, therefore the equation does not have integer roots.
Let's check the equation 1320x + 1760y = 10120. Use a calculator to find GCD(1320, 1760) = 440. Divide 10120/440 = 23. As a result, we get an integer, therefore, the Diophantine equation is solvable in integer coefficients.
Conclusion
GCD and LCM play a large role in number theory, and the concepts themselves are widely used in a wide variety of areas of mathematics. Use our calculator to calculate greatest divisors and least multiples of any number of numbers.
The material presented below is a logical continuation of the theory from the article entitled LCM - least common multiple, definition, examples, connection between LCM and GCD. Here we will talk about finding the least common multiple (LCM), And Special attention Let's focus on solving examples. First, we will show how the LCM of two numbers is calculated using the GCD of these numbers. Next, we'll look at finding the least common multiple by factoring numbers into prime factors. After this, we will focus on finding the LCM of three or more numbers, and also pay attention to calculating the LCM of negative numbers.
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Calculating Least Common Multiple (LCM) via GCD
One way to find the least common multiple is based on the relationship between LCM and GCD. Existing connection between LCM and GCD allows you to calculate the least common multiple of two positive integers using a known greatest common divisor. The corresponding formula is LCM(a, b)=a b:GCD(a, b) . Let's look at examples of finding the LCM using the given formula.
Example.
Find the least common multiple of two numbers 126 and 70.
Solution.
In this example a=126 , b=70 . Let us use the connection between LCM and GCD, expressed by the formula LCM(a, b)=a b:GCD(a, b). That is, first we have to find the greatest common divisor of the numbers 70 and 126, after which we can calculate the LCM of these numbers using the written formula.
Let's find GCD(126, 70) using the Euclidean algorithm: 126=70·1+56, 70=56·1+14, 56=14·4, therefore, GCD(126, 70)=14.
Now we find the required least common multiple: GCD(126, 70)=126·70:GCD(126, 70)= 126·70:14=630.
Answer:
LCM(126, 70)=630 .
Example.
What is LCM(68, 34) equal to?
Solution.
Because 68 is divisible by 34, then GCD(68, 34)=34. Now we calculate the least common multiple: GCD(68, 34)=68·34:GCD(68, 34)= 68·34:34=68.
Answer:
LCM(68, 34)=68 .
Note that the previous example fits the following rule for finding the LCM for positive integers a and b: if the number a is divisible by b, then the least common multiple of these numbers is a.
Finding LCM by factoring numbers
Another way to find the least common multiple is based on factoring numbers into prime factors. If you compose a product from all the prime factors of given numbers, and then exclude from this product all the common prime factors present in the expansions of the given numbers, then the resulting product will be equal to the least common multiple of the given numbers.
The stated rule for finding the LCM follows from the equality LCM(a, b)=a b:GCD(a, b). Indeed, the product of numbers a and b is equal to the product of all factors involved in the expansion of numbers a and b. In turn, GCD(a, b) is equal to the product of all prime factors simultaneously present in the expansions of numbers a and b (as described in the section on finding GCD using the expansion of numbers into prime factors).
Let's give an example. Let us know that 75=3·5·5 and 210=2·3·5·7. Let's compose the product from all the factors of these expansions: 2·3·3·5·5·5·7 . Now from this product we exclude all the factors present in both the expansion of the number 75 and the expansion of the number 210 (these factors are 3 and 5), then the product will take the form 2·3·5·5·7. The value of this product is equal to the least common multiple of 75 and 210, that is, NOC(75, 210)= 2·3·5·5·7=1,050.
Example.
Factor the numbers 441 and 700 into prime factors and find the least common multiple of these numbers.
Solution.
Let's factor the numbers 441 and 700 into prime factors:
We get 441=3·3·7·7 and 700=2·2·5·5·7.
Now let’s create a product from all the factors involved in the expansion of these numbers: 2·2·3·3·5·5·7·7·7. Let us exclude from this product all factors that are simultaneously present in both expansions (there is only one such factor - this is the number 7): 2·2·3·3·5·5·7·7. Thus, LCM(441, 700)=2·2·3·3·5·5·7·7=44 100.
Answer:
NOC(441, 700)= 44 100 .
The rule for finding the LCM using factorization of numbers into prime factors can be formulated a little differently. If the missing factors from the expansion of number b are added to the factors from the expansion of the number a, then the value of the resulting product will be equal to the least common multiple of the numbers a and b.
For example, let's take the same numbers 75 and 210, their decompositions into prime factors are as follows: 75=3·5·5 and 210=2·3·5·7. To the factors 3, 5 and 5 from the expansion of the number 75 we add the missing factors 2 and 7 from the expansion of the number 210, we obtain the product 2·3·5·5·7, the value of which is equal to LCM(75, 210).
Example.
Find the least common multiple of 84 and 648.
Solution.
We first obtain the decompositions of the numbers 84 and 648 into prime factors. They look like 84=2·2·3·7 and 648=2·2·2·3·3·3·3. To the factors 2, 2, 3 and 7 from the expansion of the number 84 we add the missing factors 2, 3, 3 and 3 from the expansion of the number 648, we obtain the product 2 2 2 3 3 3 3 7, which is equal to 4 536 . Thus, the desired least common multiple of 84 and 648 is 4,536.
Answer:
LCM(84, 648)=4,536 .
Finding the LCM of three or more numbers
The least common multiple of three or more numbers can be found by sequentially finding the LCM of two numbers. Let us recall the corresponding theorem, which gives a way to find the LCM of three or more numbers.
Theorem.
Let positive integer numbers a 1 , a 2 , …, a k be given, the least common multiple m k of these numbers is found by sequentially calculating m 2 = LCM(a 1 , a 2) , m 3 = LCM(m 2 , a 3) , … , m k = LCM(m k−1 , a k) .
Let's consider the application of this theorem using the example of finding the least common multiple of four numbers.
Example.
Find the LCM of four numbers 140, 9, 54 and 250.
Solution.
In this example, a 1 =140, a 2 =9, a 3 =54, a 4 =250.
First we find m 2 = LOC(a 1, a 2) = LOC(140, 9). To do this, using the Euclidean algorithm, we determine GCD(140, 9), we have 140=9·15+5, 9=5·1+4, 5=4·1+1, 4=1·4, therefore, GCD(140, 9)=1 , from where GCD(140, 9)=140 9:GCD(140, 9)= 140·9:1=1,260. That is, m 2 =1 260.
Now we find m 3 = LOC (m 2 , a 3) = LOC (1 260, 54). Let's calculate it through GCD(1 260, 54), which we also determine using the Euclidean algorithm: 1 260=54·23+18, 54=18·3. Then gcd(1,260, 54)=18, from which gcd(1,260, 54)= 1,260·54:gcd(1,260, 54)= 1,260·54:18=3,780. That is, m 3 =3 780.
All that remains is to find m 4 = LOC(m 3, a 4) = LOC(3 780, 250). To do this, we find GCD(3,780, 250) using the Euclidean algorithm: 3,780=250·15+30, 250=30·8+10, 30=10·3. Therefore, GCM(3,780, 250)=10, whence GCM(3,780, 250)= 3 780 250: GCD(3 780, 250)= 3,780·250:10=94,500. That is, m 4 =94,500.
So the least common multiple of the original four numbers is 94,500.
Answer:
LCM(140, 9, 54, 250)=94,500.
In many cases, it is convenient to find the least common multiple of three or more numbers using prime factorizations of the given numbers. In this case, you should adhere to the following rule. The least common multiple of several numbers is equal to the product, which is composed as follows: the missing factors from the expansion of the second number are added to all the factors from the expansion of the first number, the missing factors from the expansion of the third number are added to the resulting factors, and so on.
Let's look at an example of finding the least common multiple using prime factorization.
Example.
Find the least common multiple of the five numbers 84, 6, 48, 7, 143.
Solution.
First, we obtain decompositions of these numbers into prime factors: 84=2·2·3·7, 6=2·3, 48=2·2·2·2·3, 7 (7 is a prime number, it coincides with its decomposition into prime factors) and 143=11·13.
To find the LCM of these numbers, to the factors of the first number 84 (they are 2, 2, 3 and 7), you need to add the missing factors from the expansion of the second number 6. The decomposition of the number 6 does not contain missing factors, since both 2 and 3 are already present in the decomposition of the first number 84. Next, to the factors 2, 2, 3 and 7 we add the missing factors 2 and 2 from the expansion of the third number 48, we get a set of factors 2, 2, 2, 2, 3 and 7. There will be no need to add multipliers to this set in the next step, since 7 is already contained in it. Finally, to the factors 2, 2, 2, 2, 3 and 7 we add the missing factors 11 and 13 from the expansion of the number 143. We get the product 2·2·2·2·3·7·11·13, which is equal to 48,048.
A multiple is a number that is divisible by given number without a trace. The least common multiple (LCM) of a group of numbers is the smallest number that is divisible by each number in the group without leaving a remainder. To find the least common multiple, you need to find the prime factors of given numbers. The LCM can also be calculated using a number of other methods that apply to groups of two or more numbers.
Steps
Series of multiples
- For example, find the least common multiple of 5 and 8. These are small numbers, so you can use this method.
-
A multiple is a number that is divisible by a given number without a remainder. Multiples can be found in the multiplication table.
- For example, numbers that are multiples of 5 are: 5, 10, 15, 20, 25, 30, 35, 40.
-
Write down a series of numbers that are multiples of the first number. Do this under multiples of the first number to compare two sets of numbers.
- For example, numbers that are multiples of 8 are: 8, 16, 24, 32, 40, 48, 56, and 64.
-
Find the smallest number that is present in both sets of multiples. You may have to write long series of multiples to find the total number. The smallest number that is present in both sets of multiples is the least common multiple.
- For example, the smallest number that appears in the series of multiples of 5 and 8 is the number 40. Therefore, 40 is the least common multiple of 5 and 8.
Prime factorization
-
Look at these numbers. The method described here is best used when given two numbers, each of which is greater than 10. If smaller numbers are given, use a different method.
- For example, find the least common multiple of the numbers 20 and 84. Each of the numbers is greater than 10, so you can use this method.
-
Factor the first number into prime factors. That is, you need to find such prime numbers, when multiplied, this number is obtained. Once you have found the prime factors, write them as equalities.
- For example, 2 × 10 = 20 (\displaystyle (\mathbf (2) )\times 10=20) And 2 × 5 = 10 (\displaystyle (\mathbf (2) )\times (\mathbf (5) )=10). Thus, simple factors the numbers 20 are the numbers 2, 2 and 5. Write them as an expression: .
-
Factor the second number into prime factors. Do this in the same way as you factored the first number, that is, find such prime numbers that, when multiplied, will yield the given number.
- For example, 2 × 42 = 84 (\displaystyle (\mathbf (2) )\times 42=84), 7 × 6 = 42 (\displaystyle (\mathbf (7) )\times 6=42) And 3 × 2 = 6 (\displaystyle (\mathbf (3) )\times (\mathbf (2) )=6). Thus, the prime factors of the number 84 are the numbers 2, 7, 3 and 2. Write them as an expression: .
-
Write down the factors common to both numbers. Write such factors as a multiplication operation. As you write each factor, cross it out in both expressions (expressions that describe factorizations of numbers into prime factors).
- For example, both numbers have a common factor of 2, so write 2 × (\displaystyle 2\times ) and cross out the 2 in both expressions.
- What both numbers have in common is another factor of 2, so write 2 × 2 (\displaystyle 2\times 2) and cross out the second 2 in both expressions.
-
Add the remaining factors to the multiplication operation. These are factors that are not crossed out in both expressions, that is, factors that are not common to both numbers.
- For example, in the expression 20 = 2 × 2 × 5 (\displaystyle 20=2\times 2\times 5) Both twos (2) are crossed out because they are common factors. The factor 5 is not crossed out, so write the multiplication operation like this: 2 × 2 × 5 (\displaystyle 2\times 2\times 5)
- In expression 84 = 2 × 7 × 3 × 2 (\displaystyle 84=2\times 7\times 3\times 2) both twos (2) are also crossed out. The factors 7 and 3 are not crossed out, so write the multiplication operation like this: 2 × 2 × 5 × 7 × 3 (\displaystyle 2\times 2\times 5\times 7\times 3).
-
Calculate the least common multiple. To do this, multiply the numbers in the written multiplication operation.
- For example, 2 × 2 × 5 × 7 × 3 = 420 (\displaystyle 2\times 2\times 5\times 7\times 3=420). So the least common multiple of 20 and 84 is 420.
Finding common factors
-
Draw a grid like for a game of tic-tac-toe. Such a grid consists of two parallel lines that intersect (at right angles) with another two parallel lines. This will give you three rows and three columns (the grid looks a lot like the # icon). Write the first number in the first line and second column. Write the second number in the first row and third column.
- For example, find the least common multiple of the numbers 18 and 30. Write the number 18 in the first row and second column, and write the number 30 in the first row and third column.
-
Find the divisor common to both numbers. Write it down in the first row and first column. It is better to look for prime factors, but this is not a requirement.
- For example, 18 and 30 are even numbers, so their common factor will be 2. So write 2 in the first row and first column.
-
Divide each number by the first divisor. Write each quotient under the appropriate number. A quotient is the result of dividing two numbers.
- For example, 18 ÷ 2 = 9 (\displaystyle 18\div 2=9), so write 9 under 18.
- 30 ÷ 2 = 15 (\displaystyle 30\div 2=15), so write down 15 under 30.
-
Find the divisor common to both quotients. If there is no such divisor, skip the next two steps. Otherwise, write the divisor in the second row and first column.
- For example, 9 and 15 are divisible by 3, so write 3 in the second row and first column.
-
Divide each quotient by its second divisor. Write each division result under the corresponding quotient.
- For example, 9 ÷ 3 = 3 (\displaystyle 9\div 3=3), so write 3 under 9.
- 15 ÷ 3 = 5 (\displaystyle 15\div 3=5), so write 5 under 15.
-
If necessary, add additional cells to the grid. Repeat the described steps until the quotients have a common divisor.
-
Circle the numbers in the first column and last row of the grid. Then write the selected numbers as a multiplication operation.
- For example, the numbers 2 and 3 are in the first column, and the numbers 3 and 5 are in the last row, so write the multiplication operation like this: 2 × 3 × 3 × 5 (\displaystyle 2\times 3\times 3\times 5).
-
Find the result of multiplying numbers. This will calculate the least common multiple of two given numbers.
- For example, 2 × 3 × 3 × 5 = 90 (\displaystyle 2\times 3\times 3\times 5=90). So the least common multiple of 18 and 30 is 90.
Euclid's algorithm
-
Remember the terminology associated with the division operation. The dividend is the number that is being divided. The divisor is the number that is being divided by. A quotient is the result of dividing two numbers. A remainder is the number left when two numbers are divided.
- For example, in the expression 15 ÷ 6 = 2 (\displaystyle 15\div 6=2) ost. 3:
15 is the dividend
6 is a divisor
2 is quotient
3 is the remainder.
- For example, in the expression 15 ÷ 6 = 2 (\displaystyle 15\div 6=2) ost. 3:
Look at these numbers. The method described here is best used when given two numbers, each of which is less than 10. If given big numbers, use another method.
Second number: b=
Thousand separator Without space separator "´
Result:
Greatest common divisor GCD( a,b)=6
Least common multiple of LCM( a,b)=468
Greatest natural number, by which the numbers a and b are divided without remainder, is called greatest common divisor(GCD) of these numbers. Denoted by gcd(a,b), (a,b), gcd(a,b) or hcf(a,b).
Least common multiple The LCM of two integers a and b is the smallest natural number that is divisible by a and b without a remainder. Denoted LCM(a,b), or lcm(a,b).
The integers a and b are called mutually prime, if they have no common divisors other than +1 and −1.
Greatest common divisor
Let two positive numbers be given a 1 and a 2 1). It is required to find the common divisor of these numbers, i.e. find such a number λ , which divides numbers a 1 and a 2 at the same time. Let's describe the algorithm.
1) In this article, the word number will be understood as an integer.
Let a 1 ≥ a 2 and let
Where m 1 , a 3 are some integers, a 3 <a 2 (remainder of division a 1 per a 2 should be less a 2).
Let's pretend that λ divides a 1 and a 2 then λ divides m 1 a 2 and λ divides a 1 −m 1 a 2 =a 3 (Statement 2 of the article “Divisibility of numbers. Divisibility test”). It follows that every common divisor a 1 and a 2 is the common divisor a 2 and a 3. The reverse is also true if λ common divisor a 2 and a 3 then m 1 a 2 and a 1 =m 1 a 2 +a 3 is also divisible by λ . Therefore the common divisor a 2 and a 3 is also a common divisor a 1 and a 2. Because a 3 <a 2 ≤a 1, then we can say that the solution to the problem of finding the common divisor of numbers a 1 and a 2 reduced to the simpler problem of finding the common divisor of numbers a 2 and a 3 .
If a 3 ≠0, then we can divide a 2 on a 3. Then
,
Where m 1 and a 4 are some integers, ( a 4 remainder from division a 2 on a 3 (a 4 <a 3)). By similar reasoning we come to the conclusion that common divisors of numbers a 3 and a 4 coincides with common divisors of numbers a 2 and a 3, and also with common divisors a 1 and a 2. Because a 1 , a 2 , a 3 , a 4, ... are numbers that are constantly decreasing, and since there is a finite number of integers between a 2 and 0, then at some step n, remainder of the division a n on a n+1 will be equal to zero ( a n+2 =0).
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Every common divisor λ numbers a 1 and a 2 is also a divisor of numbers a 2 and a 3 , a 3 and a 4 , .... a n and a n+1 . The converse is also true, common divisors of numbers a n and a n+1 are also divisors of numbers a n−1 and a n , .... , a 2 and a 3 , a 1 and a 2. But the common divisor of numbers a n and a n+1 is a number a n+1 , because a n and a n+1 are divisible by a n+1 (remember that a n+2 =0). Hence a n+1 is also a divisor of numbers a 1 and a 2 .
Note that the number a n+1 is the largest divisor of numbers a n and a n+1 , since the greatest divisor a n+1 is itself a n+1 . If a n+1 can be represented as a product of integers, then these numbers are also common divisors of numbers a 1 and a 2. Number a n+1 is called greatest common divisor numbers a 1 and a 2 .
Numbers a 1 and a 2 can be either positive or negative numbers. If one of the numbers is equal to zero, then the greatest common divisor of these numbers will be equal to the absolute value of the other number. The greatest common divisor of zero numbers is undefined.
The above algorithm is called Euclidean algorithm to find the greatest common divisor of two integers.
An example of finding the greatest common divisor of two numbers
Find the greatest common divisor of two numbers 630 and 434.
- Step 1. Divide the number 630 by 434. The remainder is 196.
- Step 2. Divide the number 434 by 196. The remainder is 42.
- Step 3. Divide the number 196 by 42. The remainder is 28.
- Step 4. Divide the number 42 by 28. The remainder is 14.
- Step 5. Divide the number 28 by 14. The remainder is 0.
In step 5, the remainder of the division is 0. Therefore, the greatest common divisor of the numbers 630 and 434 is 14. Note that the numbers 2 and 7 are also divisors of the numbers 630 and 434.
Coprime numbers
Definition 1. Let the greatest common divisor of the numbers a 1 and a 2 is equal to one. Then these numbers are called coprime numbers, having no common divisor.
Theorem 1. If a 1 and a 2 coprime numbers, and λ some number, then any common divisor of numbers λa 1 and a 2 is also a common divisor of numbers λ And a 2 .
Proof. Consider the Euclidean algorithm for finding the greatest common divisor of numbers a 1 and a 2 (see above).
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From the conditions of the theorem it follows that the greatest common divisor of the numbers a 1 and a 2 and therefore a n and a n+1 is 1. That is a n+1 =1.
Let's multiply all these equalities by λ , Then
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Let the common divisor a 1 λ And a 2 yes δ . Then δ is included as a multiplier in a 1 λ , m 1 a 2 λ and in a 1 λ -m 1 a 2 λ =a 3 λ (see "Divisibility of numbers", Statement 2). Further δ is included as a multiplier in a 2 λ And m 2 a 3 λ , and, therefore, is included as a factor in a 2 λ -m 2 a 3 λ =a 4 λ .
Reasoning this way, we are convinced that δ is included as a multiplier in a n−1 λ And m n−1 a n λ , and therefore in a n−1 λ −m n−1 a n λ =a n+1 λ . Because a n+1 =1, then δ is included as a multiplier in λ . Therefore the number δ is the common divisor of numbers λ And a 2 .
Let us consider special cases of Theorem 1.
Consequence 1. Let a And c Prime numbers are relatively b. Then their product ac is a prime number with respect to b.
Really. From Theorem 1 ac And b have the same common divisors as c And b. But the numbers c And b relatively simple, i.e. have a single common divisor 1. Then ac And b also have a single common divisor 1. Therefore ac And b mutually simple.
Consequence 2. Let a And b coprime numbers and let b divides ak. Then b divides and k.
Really. From the approval condition ak And b have a common divisor b. By virtue of Theorem 1, b must be a common divisor b And k. Hence b divides k.
Corollary 1 can be generalized.
Consequence 3. 1. Let the numbers a 1 , a 2 , a 3 , ..., a m are prime relative to the number b. Then a 1 a 2 , a 1 a 2 · a 3 , ..., a 1 a 2 a 3 ··· a m, the product of these numbers is prime relative to the number b.
2. Let us have two rows of numbers
such that every number in the first series is prime in the ratio of every number in the second series. Then the product
You need to find numbers that are divisible by each of these numbers.
If a number is divisible by a 1, then it has the form sa 1 where s some number. If q is the greatest common divisor of numbers a 1 and a 2, then
Where s 1 is some integer. Then
is least common multiples of numbers a 1 and a 2 .
a 1 and a 2 are relatively prime, then the least common multiple of the numbers a 1 and a 2:
We need to find the least common multiple of these numbers.
From the above it follows that any multiple of numbers a 1 , a 2 , a 3 must be a multiple of numbers ε And a 3 and back. Let the least common multiple of the numbers ε And a 3 yes ε 1 . Next, multiples of numbers a 1 , a 2 , a 3 , a 4 must be a multiple of numbers ε 1 and a 4 . Let the least common multiple of the numbers ε 1 and a 4 yes ε 2. Thus, we found out that all multiples of numbers a 1 , a 2 , a 3 ,...,a m coincide with multiples of a certain number ε n, which is called the least common multiple of the given numbers.
In the special case when the numbers a 1 , a 2 , a 3 ,...,a m are relatively prime, then the least common multiple of the numbers a 1 , a 2, as shown above, has the form (3). Next, since a 3 prime in relation to numbers a 1 , a 2 then a 3 prime number a 1 · a 2 (Corollary 1). Means the least common multiple of the numbers a 1 ,a 2 ,a 3 is a number a 1 · a 2 · a 3. Reasoning in a similar way, we arrive at the following statements.
Statement 1. Least common multiple of coprime numbers a 1 , a 2 , a 3 ,...,a m is equal to their product a 1 · a 2 · a 3 ··· a m.
Statement 2. Any number that is divisible by each of the coprime numbers a 1 , a 2 , a 3 ,...,a m is also divisible by their product a 1 · a 2 · a 3 ··· a m.