因为：tan(a+π/4)=[tana+tanπ/4]/[1-tanatanπ/4]=(tana+1)/(1-tana)=3,解得：tana=1/2
(cosα+2sinα)/(cosα-sinα)=(1+2tana)/(1-tana)=(1+1)/(1-1/2)=4