5 Si \(n\in\mathbb N\), \(n\ge2\), demuestra que \[\dfrac{1}{\cos^2\frac{\pi}{2n}}+\dfrac{1}{\cos^2\frac{2\pi}{2n}}+\dots+\dfrac{1}{\cos^2\frac{(n-1)\pi}{2n}}=\dfrac{2}{3}(n^2-1)\]