
Andrew D. answered 06/03/15
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If the sides are a,b and diagonals are p,q then p^2+q^2=2(a^2+b^2)..there is no simple proof so I am surprised this is an SAT question. Too hard. If you want to see the proof let me know.
So p^2+q^2=170=121+49=11^2+7^2. So diagonals of length 7cm and 11cm are possible.