已知tanα=1/2,则sinαcosα-2sin^2α=
问题描述：
已知tanα=1/2,则sinαcosα-2sin^2α=
2.若sina+3cosa=0,则cosa+2sina/6cosa-3sina=
3.函数y=2sin^2θ+cosθ-1的值域是
答
sinαcosα-2sin^2α=1/2sin2α-(1-cos2α)=1/2sin2α-1+cos2α=1/2sin2α+cos2α-1=1/2*2tanα/(1+tan^2α)+(1-tan^2α)/(1+tan^2α)-1=tanα/(1+tan^2α)+(1-tan^2α)/(1+tan^2α)-1=(1/2)/(1+1/4)+(1-1/4)/(1+1/4...