For your problem:
In other words, you want to prove that:
Let , and
If you proved that
By substituting x = anynumber in the last equation you will get the value of .
Draw a right-angled triangle.
Label one acute angle A and the other B.
Label the side opposite A as x and the hypotenuse 1.
Inverse Sine and Inverse Cosine give you angle from the ratios of
For this triangle
Since the sum of the acute angles in a right-angled triangle is
your statement is proven.