已知条件上下同除以cosα 得到(tanα+3)/(3-tanα)=5解得tanα=2 所求式子等于（sin^2α-sinαcosα）/1=(sin^2α-sinαcosα)/(sin^2α+cos^2α)=(tan^2α-tanα)/(tan^2α+1)=2/5 望采纳!