bnu_dudu
幼苗
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-3/2
由于等比三角形三内角均为60
向量AB逆时针旋转180-60=120度可转化为向量CA (注意向量方向的一致性)
顺时针旋转120度可转化为向量BC
设
a=(cosx,sinx)
则
b=(cos(x-120),sin(x-120))
c=(cos(x+120),sin(x+120))
a·b=cosxcos(x-120)+sinxsin(x-120)=cos(x-(x-120))=cos(120)=-1/2
a·c=cosxcos(x+120)+sinxsin(x+120)=cos(x-(x+120))=cos(-120)=-1/2
b·c=cos(x-120)cos(x+120)+sin(x-120)sin(x+120)
=cos((x-120)-(x+120))
=cos(-240)
=cos(240)
=-1/2
a·b+a·c+b·c=-3/2
1年前
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