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If a and b are coprime and both are proved divisible by 3, what is the correct contradiction about gcd(a,b)?

Answer and explanation

Correct answer: gcd(a,b) = 1 and gcd(a,b) ≥ 3 cannot both hold

By definition, a and b being coprime means their greatest common divisor is exactly 1. If both are divisible by 3, we can write a = 3r and b = 3s for integers r and s. Thus 3 is a common divisor of a and b, and their greatest common divisor is at least 3. The proof has therefore derived gcd(a,b) = 1 from the lowest-terms assumption and gcd(a,b) ≥ 3 from divisibility. These mutually incompatible statements form the contradiction. Option A states it precisely. A gcd is never negative, it need not be zero, and it is not generally equal to the sum of the two numbers, so B, C, and D are invalid.

Related tags

Number-SystemsGcdContradictionProofProof Of Irrationality Of Square Root 2 And Square Root 3Number SystemsMathematicsClass 9 Mcq

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