I guess you have to find
given that .
We can immediately solve for :
(without knowing anything else about , we cannot determine the sign)
Then we get for free:
and we can now solve for :
Notice that we have 2*2 = 4 possible choices of sign for either sin or cos.
• If both are positive, then
• If both are negative, then
• If sin is positive and cos is negative, then
• If cos is positive and sin is negative, then