=0
sina+sinb=-siny
cosa+cosb=-cosy
sinasinb=0.5((sina+sinb)^2-1)=0.5(siny^2-1)
cosacosb=0.5(cosy^2-1)
cos(a-b)=sinasinb+cosacosb=0
若sina+sinb+siny=cosa+cosb+cosy=0,则cos(a-b)等于
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