离散数学题目求助。

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(a) Is (Z × Z, +) isomorphic to (Z, +)? Here we define (a, b) + (x, y) = (a + x, b + y).
(b) Is (Q × Q, +) isomorphic to (Q, +)? Again we define (a, b) + (x, y) = (a + x, b + y). 想了很久实在不太明白。怎么证明不是同构