1.
If f:[1,∞)→[1,∞)f : [1, \infty) \to [1, \infty) is defined by f(x)=1+1+4log⁡2x2f(x) = \frac{1 + \sqrt{1 + 4\log_2 x}}{2}, then f−1(3)=f^{-1}(3) =