2 things: check your squaring of a/cos30, and when you're sqrting (op)^2, you've got to sqrt the whole of the rhs. If you sqrt 1 side of an equation, you've got to sqrt the whole of the other side. Eg if I have 5^2=4^2+3^2 (25=16+9), 5 doesn't = 4+3, but it does = sqrt(16+9) <-- sqrt(16+9) doesn't equal sqrt(16)+sqrt(9).