【急】已知向量a=(sinθ,cosθ-2sinθ),b=(1.2)

(sinθ^2+(cosθ-2sinθ)^2=5
1+4(sinθ)^2-4sinθcosθ=5
(sinθ)^2-sinθconθ-1=0
(cosθ)^2+sinθcosθ=0
cosθ(cosθ+sinθ)=0
1 cosθ=0
θ=π/2
2 cosθ+sinθ=0
tanθ=-1
θ=3π/4