This identity is not true, I don't think. Let Then the LHS is approximately and the RHS is approximately
Take the cosine of both sides:
must be equal to x.
To check this. let
So we have
Now sketch a right triangle. Label one angle . The leg of the triangle across from has a length of and the other leg is . This means the hypotenuse has a length of .
Square both of these and subtract and you get