(sinα+π/2)[1+tan(-α)]=1/2
cosα（1-tanα）=1/2
cosα（1-sinα/cosα）=1/2
cosα-sinα=1/2
cos^2α-2sinαcosα+sin^2α=1/4
1-1/4=2sinαcosα
sinαcosα=3/8