Đẳng thức tổng 2 góc

$\sin (x+y)=\sin (x)\cos (y)+\cos (x)\sin (y)$

$\sin (x-y)=\sin (x)\cos (y)-\cos (x)\sin (y)$

$\cos (x+y)=\cos (x)\cos (y)-\sin (x)\sin (y)$

$\cos (x-y)=\cos (x)\cos (y)+\sin (x)\sin (y)$

$\sin (x)+\sin (y)=2\sin \left(\frac{x+y}{2}\right)\cos \left(\frac{x-y}{2}\right)$

$\sin (x)-\sin (y)=2\cos \left(\frac{x+y}{2}\right)\sin \left(\frac{x-y}{2}\right)$

$\cos (x)+\cos (y)=2\cos \left(\frac{x+y}{2}\right)\cos \left(\frac{x-y}{2}\right)$

$\cos (x)-\cos (y)=-2\sin \left(\frac{x+y}{2}\right)\sin \left(\frac{x-y}{2}\right)$

$\tan (x)+\tan (y)=\frac{\sin (x+y)}{\cos (x)\cos (y)}$

$\tan (x)-\tan (y)=\frac{\sin (x-y)}{\cos (x)\cos (y)}$

$\cot (x)+\cot (y)=\frac{\sin (x+y)}{\sin (x)\sin (y)}$

$\cot (x)-\cot (y)=\frac{-\sin (x-y)}{\sin (x)\sin (y)}$