Срочно Докажите тождество (1-ctg a)^2+(1+ctg a)^2= 2(sin a)^-2

[latex](1-ctg \alpha )^2+(1+ctg \alpha )^2=2(\sin \alpha )^{-2}\\ 1-2ctg \alpha +ctg^2 \alpha +1+2ctg \alpha +ctg^2 \alpha =2(\sin \alpha )^{-2}\\ 2+2ctg^2 \alpha =2(\sin \alpha )^{-2}\\ 2(1+ctg^2 \alpha )=2(\sin \alpha )^{-2}\\ 2\cdot \frac{1}{\sin^2 \alpha } =2(\sin \alpha )^{-2}\\ 2(\sin \alpha )^{-2}=2(\sin \alpha )^{-2}[/latex]