若A,B,C三角形中,则答案是2.sina^2+sinb^2+sinc^2-2cosacosbcosc=3-(cosa^2+cosb^2+cosc^2+2cosacosbcosc)=3-{cosa*[cosa+2cosb*cosc]+(1/2)*[cos(2b)+cos(2c)+2]}=3-{-cos(b+c)*[-cos(b+c)+2cosb*cosc]+(1/2)*[cos(...