Sea $\theta_{i}\in(0,\frac{\pi}{2})(i=1,2,\cdots,n)$ . Demostrar: $$(\sum_{i=1}^n\tan\theta_{i})(\sum_{i=1}^n\cot\theta_{i})\geq(\sum_{i=1}^n\sin\theta_{i})^2+(\sum_{i=1}^n\cos\theta_{i})^2.$$