Question

If A+B+C =2W, cos²W + cos²(W - A) + cos²(W – B) + cos²(W-C)
= 2 + 2 cos A cos B cos C.​

Answer 1

Answer:

We write cos2A=1−sin2A

and as in ΔABC   A+B+C=180

⟹cosC=cos(180−A−B)=−cos(A+B)