jmc

Question

The function f(x) satisfies f(x + f(x)) = 4f(x)for all x, and f(1) = 4. Find f(21).

Accepted Answer

Setting x = 1, we get f(1 + f(4)) = 4f(1), so f(5) = 16.Setting x = 5, we get f(5 + f(5)) = 4f(5), so f(21) = 64.