tan=sinθ/cosθ=3 sinθ=3cosθ
sinθ=（3√10）/10 cosθ =√10/10 或 sinθ=（-3√10）/10 cosθ =-√10/10
3sin^2θ-sinθcosθ+2
=9sinθcosθ-sinθcosθ+2
=8sinθcosθ+2
=4.4