There's a task in my math book that says:

Write the expression

\/2*(cos(3x+ π/4))

using cos3x and sin3x. (\/2 means the square root of 2, I couldn't locate the square root sign on my keyboard).

I ended up with \/2*(cos3x-sin3x), but the answer is supposed to be cos3x-sin3x. What did I do wrong?

Oh, and, sorry if my translation makes no sense. I'm Norwegian and not very used to talk about mathematics in any other language than Norwegian