Trigonometry, eliminate theta
Solution 1:
HINT:
- Notice that $x+y=2\sin\theta$ and $x-y=2\tan\theta$.
- Recall that $\tan\theta\equiv\frac{\sin\theta}{\cos\theta}$
- Recall that $\sin^2\theta + \cos^2\theta \equiv 1$
- Consider $(x+y)^2$ and $(x-y)^2$ in terms of $\theta$