1/cosα-tanα=1/cosα-sinα/cosα=(1-sinα)/cosα
1/tan（π/4+α/2）=1/(sin（π/4+α/2）/cos（π/4+α/2）)=cos（π/4+α/2）/sin（π/4+α/2)=2cos（π/4+α/2）^2/(2sin（π/4+α/2)cos（π/4+α/2）)=(1+cos(π/2+α))/sin（π/2+α/2)=(1-sinα)/cosα