Question

tan A+ tan B + tan C=tan A. tan B. tan C then (A) A,B,C must be angle of triangle. (B) A+B+C must be n integral multiple of π (C) the sum of any two of A,B,C is equal to third

Answer 1

The answer is (A) . Because when the angles are in a triangle, we can assume that
$a \: + \: b \: + \: c \: = \: 180$
The rest you'll find in the attached picture(Forgive my bad handwriting)