1.
Let x≠0x \ne 0, ∣x∣<12|x| < \frac{1}{2} and f(x)=1+2x+4x2+8x3+…f(x) = 1 + 2x + 4x^2 + 8x^3 + \ldots. Then, f−1(x)=f^{-1}(x) =