Đẳng thức tổng 2 góc

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

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

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

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

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

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

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

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

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

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

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

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