Anytime you're taking a trigonometric function of a constant with a fraction of pi in it, it is a good idea to check your unit circle. There are a few angles which are extremely common, pi/2 being one of them, you will see these angles all the time, because they come out cleanly (well, i guess if you consider square root of 3, divided by 2 clean lol). Anyway, many equations use these angles, and much of the time it is necessary to be able to convert them.
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.