tan(x/2)-1/tan(x/2)=sin(x/2)/cos(x/2)-cos(x/2)/sin(x/2)=sin²(x/2)/sin(x/2)cos(x/2)-cos²(x/2)/sin(x/2)cos(x/2)=[sin²(x/2)-cos²(x/2)]/[sin(x/2)cos(x/2)]=-cosx/[(sinx)/2]=-2/tanx证毕...