Question

Determine the open intervals on which the graph of the function y=5x-tan4x, (-pi*/8, pi*/8) is concave upward or concave downward.

Answer 1

First find the 2nd derivative, set it equal to 0, and solve for x. What do you get?

Answer 2

The 2nd derivative was -8secx^2(tanx)

Answer 3

Then I don't know what to do