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Four integers from three pairwise products

Question

Integers a,b,c,da,b,c,d, each greater than 11, satisfy ab=21ab=21, bc=15bc=15 and cd=45cd=45. What is a2+b2+c2+d2a^2+b^2+c^2+d^2?

Short answer

164. bb divides both 2121 and 1515 and exceeds 11, so b=3b=3. Then a=7a=7, c=5c=5 and d=9d=9, and 49+9+25+81=16449+9+25+81=164.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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