Thanks
Just substitute $\displaystyle \tfrac{\sin\theta}{\cos\theta}$ for $\displaystyle \tan\theta$ and $\displaystyle \tfrac{\cos\theta}{\sin\theta}$ for $\displaystyle \cot\theta$. After cancelling the denominators on the left side of the equation you get $\displaystyle \sin^2\theta+\cos^2\theta=1$, which is surely true for all $\displaystyle \theta$.