Question

If A + B + C =180º, prove that cos2A + cos2B + cos2C = -1 - 4cos A cosB cosC​

Answer 1

Explanation:

if A+B+C=180°

THEN

cos2A+cos2B+cos2C=-1-4cosAcosBcosC

let us take L.H.S

cos2A+cos2B+cos2C

cos^2A-sin^2A+cos 2B+cos2C

cos^2A-(1+cos^2A)+cos2B+cos2C

thai equal to R.H.S

hence proved

thank you