2=secθ-tgθ=1/cosθ -sinθ/cosθ=(1-sinθ)/cosθ
==> 1-sinθ=2cosθ
==> 1-2sin(θ/2)cos(θ/2)=2cosθ
==> [sin(θ/2)]^2 +[cos(θ/2)]^2-2sin(θ/2)cos(θ/2)=2*{[cos(θ/2)]^2 -[sin(θ/2)]^2}
[sin(θ/2)-cos(θ/2)]^2 =2*[sin(θ/2)+cos(θ/2)][cos(θ/2)-sin(θ/2)]
==> sin(θ/2)-cos(θ/2) = 0
or : 3sin(θ/2)+cos(θ/2)=0
==> tg(θ/2)=0 or tg(θ/2)=-1/3
tg(θ/2)=0时：tgθ、secθ无意义 ==> 无解
tg(θ/2)=-1/3时：sinθ=-3/5、cosθ=4/5
因此，sinθ=-3/5、cosθ=4/5
。
