1.$\sin2\alpha=\frac{2 \tan \alpha}{1+\tan ^{2} \alpha}$.
2.$\cos2\alpha=\frac{1-\tan^{2}\alpha}{1+\tan^{2}\alpha}$.
3.$\sin^{2}\alpha=\frac{tan^{2}\alpha}{1+tan ^{2} \alpha}$.
4.$\cos^{2}\alpha=\frac{1}{1+tan ^{2} \alpha}$.