SinA + SinB + SinC = 4cos(A/2)cos(B/2)cos(C/2)VT = SinA + SinB + SinC = 2Sin[(A+B)/2].Cos[(A-B)/2] + 2Sin(C/2).Cos(C/2)VT = 2Cos(C/2).[Cos(A-B)/2 + Cos(A+B)/2 ]VT = 4Cos(C/2).Cos(A/2).Cos(B/2) = VP (ĐPCM)
SinA + SinB + SinC = 4cos(A/2)cos(B/2)cos(C/2)VT = SinA + SinB + SinC = 2Sin[(A+B)/2].Cos[(A-B)/2] + 2Sin(C/2).Cos(C/2)VT = 2Cos(C/2).[Cos(A-B)/2 + Cos(A+B)/2 ]VT = 4Cos(C/2).Cos(A/2).Cos(B/2) = VP (ĐPCM)