Đẳng thức góc đôi

• $\sin(2x)=2\sin(x)\cos(x)$
• $\cos(2x)=\cos ^{2}(x)-\sin ^{2}(x)$
• $\tan(2x)={\frac {2\tan(x)}{1-\tan ^{2}(x)}}$
• $\cos ^{2}(x)={\frac {1+\cos(2x)}{2}}$
• $\sin ^{2}(x)={\frac {1-\cos(2x)}{2}}$