Problem

Problem

If ∑k=1nf(a+k)=16(2n−1)\sum_{k=1}^n f(a+k) = 16(2^n - 1), where f(x+y)=f(x)f(y)f(x+y) = f(x)f(y) for all x,y∈N,f(1)=2x, y \in N, f(1) = 2, then f(a)=f(a) = (a) 8 (b) 6 (c) 4 (d) 2

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