学着kk
幼苗
共回答了127个问题 举报
∵a b都是锐角
sin(a-b)=3/5
cos(a-b)=4/5
cos(a+b)=5/13
sin(a+b)=12/13
cos2a=cos(a+b+a-b)
=cos(a+b)cos(a-b)-sin(a+b)sin(a-b)
=5/13*4/5-12/13*3/5
=-16/65
cos2b=cos(a+b-(a-b))
=cos(a+b)cos(a-b)+sin(a+b)sin(a-b)
=5/13*4/5+12/13*3/5
=56/65
1年前
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