Question

How do you find `cos^-1(-sqrt2/2)`?

Answer 1

`cos^-1(-sqrt2/2) = (3pi)/4`

Explanation:

Think about the unit circle -- "Where is cosine equal to `-sqrt2/2`?". This comes up with values: `(3pi)/4` and `(5pi)/4`, but because the domain of `cos^-1` is restricted to `[0,pi]`, we only list `(3pi)/4` in the answer.