∵cos(a+π/4)=4/5 180°＜a＜270°∴270°＜a+π/4＜360°∴sin(a+π/4)=-3/5tan(a+π/4)=-3/4(tana+1)/(1-tana)=-3/41-tan²a=-3/4tan²a=7/4a在第三象限tana＞0∴tana= √7/2tan2a=2tana/(1-tan²a) =2x√7/2/(1-7/4)=-3√7/4
