It's a previous exam question using compound angle forumlas. Ive tried everything to try and get the answer involving rearrangments! I know the forumlas i just cant get them to work to show this. I assume its something to do with adding or subtracting the forumla away from each other, but ive tried it. The furthest i got was this: cos (alpha) + (root 3)sin (alpha) but i cant get it into the format of what they want Here's the question :) http://img251.imageshack.us/my.php?image=mathelpsk7.jpg Thanks!