I have been running through this problem but can't get anywhere near it:
I start off:
multiply top and bottom by to give the numerator. Whats left is to prove the denominator:
I have tried whatever I can think of, double angle identities,product to sum and even a messy Euler exponential expansion, but fail... is there something fundamental I have missed here? Maybe this should be in the trig sub forum.....
Thanks for reading!