tan(a-π)
=tana
=-√5
a∈(-π/2,0)
sina0
sina/cosa=-√5
sin²a+cos²a=1
5cos²a+cos²a=1
cos²a=1/6
cosa=√6/6
(2)
tana=t/3
tan(a+π/4)=4/(2t)=2/t
tan(a+π/4)
=(tana+1)/(1-tana)
把tana=t/3代入
=（t/3+1)/(1-t/3)
=2/t
解方程
t^2+5t-6=0
(t+6)(t-1)=0
t=-6或t=1