I am not sure about my solution, if itīs done "in the right way". Anyway, here it comes:
Now raising both sides to the second power...
Looks right to me. But here's another way, just in case:
Draw a right triangle. Label one of the acute angles y. Since
Label the opposite side x and the adjacent side 1. Find the hypotenuse (s) by using the Pythagorean theorem:
Cosine is adjacent over hypotenuse, so
Squaring both sides gives you