tan(a-pi/4)=7
(tana-tanpi/4)/(1+tana*tanpi/4)=7
(tana-1)/(1+tana)=7
tana-1=7+7tana
6tana=-8
tana=-4/3
sina/cosa=-4/3
-4cosa=3sina
又a是第二象限角,则cosa