k=(2sin^2a+sin2a)/(1+tana)
＝2sinacosa(sina+cosa)/(cosa+sina)
=2sinacosa=sin2x≤1
1-k=1-2sinacosa
sina-cosa=±√(1-k)
sin(a-∏/4)
=√2/2(sina-cosa)
==±√[2(1-k)]/2