# Trigonometry - Double angle formulas

1. $sin2 \alpha=2sin \alpha.cos\alpha$

2. $cos2\alpha=cos^2\alpha-sin^2\alpha=1-2sin^2\alpha=2cos^2\alpha-1$

3. $tan2\alpha= {2tan\alpha \over {1-tan^2\alpha}}={2 \over cot\alpha -tan\alpha}$

4. $cot2\alpha={{cot^2\alpha -1} \over 2cot \alpha } = {cot\alpha - tan \alpha \over 2 }$