0＜a＜π,sina＞0,sina^2+cosa^2=1,sina-cosa=1/5,(sina-cosa)^2=1/25,sin2a=2sinacosa=24/25cos2a=cosa^2-sina^2=-7/25sin(2a-π/4）=sin2acosπ/4-cos2asinπ/4= 31√2/50