If A, B and C are angles of a triangle, then which of the following is correct?
1
sin 2A + sin 2B - sin 2C = 4 cos A sin B cos C
2
cot A cot B + cot B cot C + cot C cot A = -1
3
\(\rm \tan\frac{A}{2}\tan\frac{B}{2}+1=\tan\frac{B}{2}\tan\frac{C}{2}+\tan\frac{C}{2}\tan\frac{A}{2}\)
4
tan A + tan B + tan C = tan A tan B tan C