Question

A+B+C=180° So cos 2A+cos 2B+cos 2C=?​

Answer 1

Answer:

cos2A + cos2B + cos2C = 2cos(A+B).cos(A-B) + 2cos^2C -1  

=2cos(180-C).cos(A-B) + 2cos^2C -1  

= - 2cosC.cos(A-B) +2cos^2C -1  

= -2cosC(cos(A-B) - cosC) -1  

= -2cosC(cos(A-B) - cos(180-(A+B)) ) -1  

= -2cosC(cos(A-B) + cos(A+B)) -1  

= -2cosC( 2cosA.cosB) -1  

= -4cosAcosBcosC -1...

Step-by-step explanation: