Question

5. In a A ABC, prove that:
(i) cos (A + B) + cos C = 0​

Answer 1

Explanation:

we know that in a triangle by angle sum property

sum of angles = 180•

so for A,B and C

A+B+C=180°

so,

A+B=180-C -----(1)

Cos(A+B)+CosC=0

LHS

Cos(180-C)+CosC. [ FROM (1) ]

-CosC+CosC

0=RHS

HENCE PROVED

HOPE IT HELPS DEAR