Question

How do you find the exact value of cos 5pi/4?

Answer 1

`cos((5pi)/4)= -1/sqrt(2) or -sqrt(2)/2`

Explanation:

`(5pi)/4` is an angle in Quadrant III and as such (based on CAST) its `cos` is negative.

`(5pi)/4=pi+pi/4`
So its reference angle is `pi/4` which is a standard angle with `cos(pi/4)=1/sqrt(2)`