cost（A+45)=3/5,A在第二三象限，求cos(2A+45)

来源：百度知道编辑：UC知道时间：2024/06/08 03:39:36

-(31倍的根号2)/50.
简析：
因为A在二三象限，cos(A+45度)>0,所以cos(A+45度)在第四象限，得A的范围为(225，270) (此处只需考虑0到360). 用二倍角公式求cos(2A+90)=-7/25.另外由A的范围得(2A+90)在第三象限，故sin(2A+90)=-24/25.
cos(2A+45)=cos((2A+90)-45),用公式展开即得结果。

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