**The greatest common** ( GCD ) of two numbers is the biggest number by which both these two numbers are divisible without a remainder.

**Ways to find GCD:**

- finding all possible divisors of two numbers and choosing the largest of them
- to divide two numbers on prime factors and multiply general of them

**The least common multiple** ( LCM ) of two numbers is the least number which divides on both these two numbers without a remainder.

**Ways to find LCM:**

- to write out the first multiples of two numbers and then choose out of them the least number
- divide into prime factors number for which you find LCM, then you take multiples from first division, add multiples from second division which are not included in the first and multiple them

Understanding GCD, LCM and knowledge of the signs of divisibility is necessary in work with fractions.