sin(π/4－α)=根号2/2cosα-根号2/2sinα
=根号2/2(cosα-sinα)
=2/3
cosα-sinα=2根号2/3
(cosα-sinα)^2=8/9
1-2sinαcosα=8/9
1-sin2α=8/9
sin2α=1/9