已知tanα=-1/2,求值

题目

已知tanα=-1/2,求值
1.(sinα+cosα)/(sinα-2cosα)
2.(sin^2α-2sinαcosα-4cos^2α)/(3cos^2α-5sin^α)
3.1+sin^2α-3sinαcosα

答案

tanα=sinα/cosα=-1/2cosα=-2sinα(sinα+cosα)/(sinα-2cosα)=(sinα-2sinα)/(sinα-4sinα)=(sinα)/(-3inα)=-1/3(sin^2α-2sinαcosα-4cos^2α)/(3cos^2α-5sin^α)=(sin^2α+4sin^2α-4cos^2α)/(3cos^2...