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(a) Prove: for any positive integers a, b, d, if d | a and d | b, then d | GCD(a, b).
(a) Prove: for any positive integers a, b, d, if d | a and d | b, then d | GCD(a, b).
(b For all integers a, b, c, e, prove that the Diophantine equation ax + by + cz = e has a solution if and only if GCD(a, b, c) | e.