Hi there!

Im a little stuck on one of my questions and was hoping for a little bit of help.. So first, the question

Convert from polar to cartesian form, using double angle formula:

Ok, so things that I know:

The double angle formula which is reffered to, i believe is

But i could be wrong

The furthest i have got to in the problem so far is

And now im greatful if anyone is able to help me out please?

Thanks in advance!