I asked a question earlier about open and closed intervals, so why did my teacher say this was wrong?

Determine any values of c in the interval [0,2pi] for which f'(x) = 0.

f(x) = cos x

My answer was:

f'(x) = -sin x

set equal to 0

-sin x = 0

x = 0, pi, 2pi

she said

0/7 points: 0 and 2pi are not acceptable answers

## Comments