This identity is not true, I don't think. Let Then the LHS is approximately and the RHS is approximately
Hang on. This is going to be a ride...
Take the cosine of both sides:
So
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 .
So
and
Square both of these and subtract and you get
as advertised.
-Dan