I've been asked the prove that
by using the double-angle for and in turn.
I'm stuck at the first section of the solution given for the question, I think if I can understand this first part I will be OK after that. The first part of the solution is:
From the double-angle formula for , with , we have
Where has all this come from?
The only formula I've been given for the double-angle formula for is
Any help clearing this up would be greatly appreciated.