sin(a+b)=4/5cos(a-b)=12/13求cos 2a=?
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sin(a+b)=4/5cos(a-b)=12/13求cos2a=?sin(a+b)=4/5cos(a-b)=12/13求cos2a=?sin(a+b)=4/5cos(a-b)=12/13求cos2a
sin(a+b)=4/5cos(a-b)=12/13求cos 2a=?
sin(a+b)=4/5
cos(a-b)=12/13
求cos 2a=?
sin(a+b)=4/5cos(a-b)=12/13求cos 2a=?
上面的解答有问题!
sin(a+b)=4/5 ;cos(a+b)=3/5或-3/5;
cos(a-b)=12/13 ;sin(a-b)=5/13或-5/13;
所以:
cos(2a)=cos((a+b)+(a-b))=cos(a+b)cos(a-b)-sin(a+b)sin(a-b) =16/65或-56/65或56/65或-16/65
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