Trong $\Delta ABC$ hãy cm các đẳng thức sau:a) $\sin (A+B) =\sin C$
b)$\cos C=-\cos(A+B)$
c)$\sin \frac{A+B}{2}=\cos C$
d)$\cot A+\cot B=\frac{\sin (A+B)}{\sin A\sin B}$
e)$\sin 2A+\sin2B+\sin2C=4\sin A\sin B\sin C$
f)$\sin A+\sin B+\sin C=4\cos A/2\cos B/2\cos C/2$
g)$\cos2A+\cos 2B+\cos 2C=-1-4\cos A\cos B\cos C$
h)$\cos A+\cos B+\cos C=1+4\sin A/2\sin B/2\sin C/2$
i)$\sin^2A+\sin^2B+\sin^2C=2+2\cos A\cos B\cos C$