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I'm not sure on how to find the GCD (other that trial and error) when you are given three numbers.

Example:

12.3:find the greatest common divisor adn the least common multiple of the given set of integers

{36,-60,90}

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Approach 1: First find the GCD of the first two using the Euclidean Algorithm; call it **a**; and then find the GCD of **a** and the third number using the same algorithm. Here is the whole story with the numbers you give. (Note that you may ignore "-" since (-a,b) is the same as (a,b)).

90=(1)(60)+30

60=(2)(30)+0

So (90,60)=30.

Next, find (36,30):

36=(1)(30)+6

30=(5)(6)+0

So (36,30)=6. Consequently the GCD of 36, 60 and 90 is 6. So, the same is true for the GCD of 36, -60 and 90

Approach 2. Use the fundamental thereom of arithmetic. Roughly, factor all three numbers as product of primes and use the primes "common" to all three.