If `A+B+C=2S`, prove that : `cos^2 S + cos^2 (S-A) + cos^2 (S-B) + cos^2 (S-C) = 2+2cosA cosB cosC`.

If A+B +C =2S, prove that : cos^2 S + cos^2 (S- A) + cos^2 (S-B)+ cos^2 (S -C) = 2 +2 cos A cos B cos C .

Prove that : 2cosA cosB = cos (A + B) + cos (A-B) .

If A+B+C=2S , " then " cos^(2)S + cos^(2)(S-A)+cos^(2)(S-B)+cos^(2)(S-C)=

cos^2 A + cos^2 B +cos^2 C=1+2cosA cosB cosC .

If A+B+C=180^0 , prove that : cos^2 A + cos^2 B + cos^2 C + 2cosA cosB cosC=1 .

If A+B+C=0 , Prove : cos^2 A + cos^2 B +cos^2 C=1+2cosA cosB cosC .

If A+B+C=0 , Prove : cos^2 A + cos^2 B +cos^2 C=1+2cosA cosB cosC .

If A + B + C =180^@ , prove that : cos^2 A+ cos^2 B+cos^2 C=1-2 cos A cos B cosC .

A + B + C = 2S then prove that cos (SA) + cos (SB) + cos (SC) + cos S = 4 (cos A) / (2) (cos B) / (2) (cos C) / (2)

If A + B + C =360^@ , prove that : 1-cos^2 A-cos^2 B-cos^2 C+2 cos A cosB cos C=0 .