Question

If $A+B+C=\pi$, then what is cos (A + B) + cos C equal to ?

• A. 0
• B. 2 cos C
• C. cos C - sin C
• D. 2 sin C

Answer 1

Answer: (A)

0

$A+B+C=\pi \Rightarrow A+B=\pi -C$

Taking cos on both sides, we get

$\cos (A+B)=\cos (\pi -C)$

$\Rightarrow \cos (A+B)=-\cos C$

Substituting in given equation, we get

$\cos (A+B)+\cos C$

$=-\cos C+\cos C=0$

Hence, A is correct.