若x+y=π/3,则sinx*siny的最小值为A-1 B-0.5 C-0.75 D0.25答案为C,怎么算？

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解:x+y=π/3,积化和差sinx*siny=(-1/2)[cos(x+y)+cos(x-y)]=(-1/2)[cos丌/3+cos(x-y)]=(-1/2)[(1/2)+cos(x-y)]=-1/4-(1/2)cos(x-y)≥-1/4-1/2=-3/4=-0.75选C