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1. (a) Let a, b and c be positive integers. Prove that gcd(ac, bc) = c...

1. (a) Let a, b and c be positive integers. Prove that gcd(ac, bc) = c x gcd(a, b). (Note that c gcd(a, b) means c times the greatest common division of a and b)

(b) What is the greatest common divisor of a − 1 and a + 1? (There are two different cases you need to consider.)

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