1. The problem statement, all variables and given/known data Prove that the coordinates of the point (x',y') where the counter-clockwise rotation through the angle @ around (0,0) brings the given point (x,y) are x' = xcos@ - ysin@ y' = xsin@ + ycos@ Hint: show that for the points (x,y) = (1,0) and (x,y) = (0,1) directly, and use the fact that the vector (x,y) is equal to the combination x.(1,0) + y.(0,1) 2. Relevant equations For vectors u and v, angle @ between them u.v = |u||v|Cos@ 3. The attempt at a solution I don't want to be told how to do it, I would prefer if someone would kind of tease the solution out of me, if you know what i mean.. I've included a diagram, showing my interpretation of the question. I've tried a few different approaches for the question. I used the fact that tan@ = (m1 - m2)/(1 +m1m2). I got the slopes of the lines being y/x and y'/x'. When I plugged everything in and rewrote tan as sin/cos, I got the required formulae, but they were both being divided by each other. I also used the dot product, put this just resulted with a lot of squares which doesn't help. I don't entirely understand the hint also.