已知sinα +sinβ+ siny=0,cosα +cosβ +cosy=0,求cos(α-β)
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已知sinα +sinβ+ siny=0,cosα +cosβ +cosy=0,求cos(α-β)
已知sinα +sinβ+ siny=0,cosα +cosβ +cosy=0,求cos(α-β)
已知sinα +sinβ+ siny=0,cosα +cosβ +cosy=0,求cos(α-β)
sinr=-(sina+sinb)
cosr=-(cosa+cosb)
1=sin^2r+cos^2r
=(sina+sinb)^2+(cosa+cosb)^2
=sin^2a+2sinasinb+sin^b+cos^2a+2cosacosb+cos^2b
=2+2(sinasinb+cosacosb)
=2+2cos(a-b)
2cos(a-b)=-1
cos(a-b)=-1/2
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