sin(π/4-α)=3/5,
即 sin(π/4)cosα-cos(π/4)sinα=3/5
√2/2cosα-√2/2sinα=3/5
cosα-sinα=3/5*√2
又 sin2a=2sina cosa
=-1+ 2sina cosa +1
=-((sina)^2+(cosa)^2)+ 2sina cosa +1
=-(cosα-sinα)^2+1
=-(3/5*√2)^2+1
=-18/25+1= 7/25