Question

How do you evaluate `cos(-pi/4)`?

Answer 1

The easiest way is to look at the cosine graph.

Explanation:

I used Desmos graphing calculator to make this image (it's a pretty good calculator that does a lot more than a lot of others - and it's free!).

As you can see, when 'x' is `-pi/4`, 'y' is `sqrt2/2`.

With questions like this, it's easiest just to see how the point chosen fits into the cosine function. That way, you can use it for future reference and understand how it works.