合聚咖

合聚咖

若sina+sinb=1,则cos(a-b)等于多少

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sinasinb=(1/2)[cos(a-b)-cos(a+b)]=1

-1<=cos(a-b)<=1

-1<=cos(a+b)<=1

cos(a-b)-cos(a+b)=2

只能在cos(a-b)=1,cos(a+b)=-1时成立