#### Answer

a) $ f(x) = x^2 $
c) $ f(x) = x ^\pi$
f) $ f(x) = x^\frac{-4}{5} $

#### Work Step by Step

The power rule applies only when a variable is raised to a constant power.
We rule b) $ f(x) = 2\pi$ and d) $f(x) = \pi^x$ out because the value being raised is a constant.
We rule e) $f(x) = x^x $ out because the variable is not being raised to a constant power.