How to proof ,
thanks
cannot be proved because it is not true. I think that what you meant to ask is how to solve that equation for
Try squaring both sides and expanding:
Letting , you have
Solve for y by inspection. I see one obvious value of y such that
Then find when equals that value of y and you're done.
Note: there is more than one solution. In fact, there are infinitely many solutions to this problem because sine is a periodic function.