3sin^2α-sinαcosα+2=(3sin^2α-sinαcosα+2)/1=(5sin^2α-sinαcosα+2cos^2a)/(sin^2a+cos^2a) 分子分母同时除以 cos^2a 得=(5tan^2a-tana+2)/(tan^2a+1) tanα=3代入=（45-3+2）/(9+1)=22/5