Đẳng thức góc đôi

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