If `A+B+C+D = 2pi`, prove that : `cosA +cosB+cosC+cosD=4 cos( (A+B)/2) cos((B+C)/(2) )cos( (C+A)/2)`

If A+B+C=pi , prove that : cosA + cosB-cosC=4cos(A/2) cos(B/2) sin(C/2) -1

If A + B + C + D = 2pi , prove that: cos A + cos B + cos C + cos D =-4"cos"(A+B)/(2)"cos"(A+C)/(2)"cos"(A+D)/(2)

If A+B+C+D=2pi , show that : cosA-cosB+cosC-cosD=4sin( (A+B)/(2)) sin( (A+D)/(2)) cos( (A+C)/(2)) .

If A+B+C = pi , prove that : cos2A +cos2B +cos2C =-1-4cosAcosBcosC .

If A+B+C = pi , prove that : cos2A-cos2B -cos2C = -1+4cosAsinBsinC

If A+B+C = pi prove that cos 4A + cos 4B + cos 4C = -1 + 4 cos 2A cos 2B cos 2C

If A+B+C = pi , prove that : cosA- cosB - cosC = 1-4sinA//2cosB//2cosC//2 .

Show that : cos A+ cosB + cosC+ cos(A+B+C) = 4cos""(B+C)/(2)cos""(C+A)/(2)cos""(A+B)/(2) .

If A+B+C=pi , prove that : cos2A+cos2B-cos2C=1-4sinA sinB cosC

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