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ChallengeSuperchallenge

Challenge · Superchallenge problem

Let F1=F2=1,F3=2,…F_1 = F_2 = 1, F_3 = 2, \ldots be the Fibonacci numbers. Without computing any enormous products, evaluate

F152−F10⋅F20.F_{15}^2 - F_{10} \cdot F_{20}.
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