Hello, Furyan!
Can someone help me with this please?
. .
You need this identity: .
Here's how that identity is derived.
First a proof of the angle sum formula
Ans 1
Now, using the angle sum formula with we have
And if you're trying to use integration by parts...
And with a little manipulation I'm sure you can get this to be the same form as your answer you gained using the double angle identity...
Dear Prove It
I have been looking at the double angle identity for and, for some reason, was having trouble realising what you had pointed out in your original post. Now it's crystal clear. Some how I keep missing the elementary principles.
I will try and find the integral again using the substitution you have suggested.
Thank you also for the help with using integration by parts. I'd never have been able to do that, but I am very interested in it and and will work through it until I understand it.
Thank you very much for the time you have taken to help me. It's extremely generous of you and very grateful.