Question

answer the question

Answer 1

A+B = 180 - c

(tan A + tan B ) / (1- tanA.tanB) = - tan c

or tan A + tan B + tan C = tan A . tan B . tan C

divided both sides by tan A tan B tan C

1/tanB 1/tanC + 1/tanA .1/tanC + 1/tanA + 1/tanB = 1

or cot B cot C + cot A cot C + cot A cot B = 1.