secx-tanx=1/cosx-sinx/cosx=(1-sinx)/cosx
=(coxx/2-sinx/2)^2/(cosx/2)^2-(sinx/2)^2
=(coxx/2-sinx/2)/（coxx/2-sinx/2)
tan(π/4 - x/2)=(1-tanx/2)/(1+tanx/2)
=(coxx/2-sinx/2)/（coxx/2-sinx/2)
∴secx - tanx = tan(π/4 - x/2)