a,b是锐角,cos(a+b)=3/5,sin(a-b)=3/4,cos2a=
问题描述：
a,b是锐角,cos(a+b)=3/5,sin(a-b)=3/4,cos2a=
答
cos(a-b)=√7/4
sin(a+b)=4/5
cos2a=cos((a+b)+(a-b))=cos(a+b)cos(a-b)-sin(a+b)sin(a-b)=-3/5+3√7/20