I need to find the critical numbers for the following problems:

I got an answer for the first few but I don't think it's corrects.

- f'(x) = 5x^4-6x^2+1
- f'(x) = 2x^(-1/3)-1
- f'(t) = -sin t, -2(pi)<=t<=2(pi)
- f'(x) = (-2x(x+2))/(x-2)^3
- f'(x) = e^x/(1+e^x)^2

for 1:

0

for 2:

8

At this point I figured I was probably doing something wrong. Could someone please walk me through how to do these problems? I'd really appreciate it.