Question

Apparently cos((9 * pi) / 4) = cos((pi / 4) + 2 * pi)

Can someone explain why this is?

Answer 1

This is because 9pi/4 is equal to pi/4 + 2pi... try it by putting the 2pi over a common denominator:

\[\large \frac{ \pi }{ 4} + 2 \pi = \frac{ \pi }{ 4} + \frac{ 8 \pi }{ 4} = ?\]

Answer 2

yes...^^^
and also, 9pi/4 and pi/4 are coterminal angles.... that is, the terminal side is the same position for both angles.