Sorry about that, hopefully this helps you.
S_{n} = \tan\theta\tan\2\theta+\tan2\theta\tan3\theta+\tan3\theta\tan4\theta+...+\tan{n}\theta\tan((n+1)\theta)\\\\\text{using the expression}\\\\tan(a\theta)\tan((a+1)\theta) = \frac{1}{\\tan\theta}(\tan((a+1)\theta) - \tan{a}\theta)-1\\\\S_{n} =...