在△ABC中,下列等式一定成立是A.sin(2A+2B)=-cos(2c)B.sin(A+B)=-sinC, C.sin(A+B/2)=-cos

问题描述:

在△ABC中,下列等式一定成立是A.sin(2A+2B)=-cos(2c)B.sin(A+B)=-sinC, C.sin(A+B/2)=-cos
C.sin(A+B/2)=-cosC /2d.sin(A+B/2)=COSC/2

sin(2A+2B)=sin[2(180°-C)]=sin[360°-2C]=sin[-2C]=-sin(2C)
sin(A+B)=sin(180°-C)=sin(-C)=-sin(C)
C,D选项为简单干扰项,均排除