这样
tanα=sinα/cosα=-3, sinα=-3cosα, sin^2α=9cos^2α=9-9sin^2α, sinα=±3√10/10cosα=±√10/10α∈Ⅱ 时：sinαcosα=3√10/10X(-√10/10)=-3/10α∈ Ⅳ时: sinαcosα=(-3√10/10)X√10/10=-3/10
