Question

If `sinalpha=4/5` and `alpha` is obtuse angle, what is `cosalpha`?

Answer 1

`cosalpha=-3/5`

Explanation:

As `alpha` is obtuse, it lies in second quadrant and `cosalpha` will be negative.

Now `sinalpha=4/5` and therefore

`cosalpha=-sqrt(1-(4/5)^2)=-sqrt(1-16/25)=-sqrt(9/25)=-3/5`