cosa-sina=-√5/5 sina=√5/5+cosa
cos^2a+sin^2a=1
2cos^2a+2√5cosa/5-4/5=0
cos^2a+√5cosa/5-2/5=0
cosa=√5/5或cosa=-2√5/5 0＜a＜π/2 cosa>0
cosa=√5/5
sina=2√5/5
tana=2
(2cosasina-cosa+1)/(1-tana)
=(4/5-√5/5+1)/(-1)
=√5/5-9/5