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Prove: If n≡3 (mod 8) and n=a^2+b^2+c^2+d^2, then exactly one of a, b, c, d is...

Prove: If n≡3 (mod 8) and n=a^2+b^2+c^2+d^2, then exactly one of a, b, c, d is even. (Hint: What can each square be modulo 8?)

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