Question

if A+B+C=180,find cos (A+B) +cos C..?

Answer 1

Answer:

1/2 cos(A+B+C)

Step-by-step explanation:

cos (A+B) + cos C

= cos (A+B) cos C + sin (A+B) sin C

= 1/4 [cos (A+B+C) + cos (A+B-C) + cos (A+B-C) - cos (A+B+C)]

= 1/4 x 2 cos (A+B-C)

= 1/2 cos (A+B-C)