sina=3/5,a∈(pai/2,pai), ∴ cosa=-4/5
sin(a-pai/4)=1/根号(2)*(sina-cosa)=7根号(2)/10
tan2a=sin2a/cos2a=2*sina*cosa/(cos^2a-sin^2a)=-24/7
sina=3/5,a∈(pai/2,pai), ∴ cosa=-4/5
sin(a-pai/4)=1/根号(2)*(sina-cosa)=7根号(2)/10
tan2a=sin2a/cos2a=2*sina*cosa/(cos^2a-sin^2a)=-24/7