I was having trouble proving this and was wondering if anyone could help me out... thanks in advance.
Thanks for the help! I was wondering if you could tell me which rule you used in the first line of working i.e. ?
I eventually got it to work using the addition formula:
which gives the same result as yours but after about 2 pages of working - do you know a quicker way? Or did you just omit your working? Thanks again.