1+cos2x=2(cosx)^2
tanx/2-1/(tanx/2)=(sinx/2)/(cosx/2) - (cosx/2)/(sinx/2)
=[(sinx/2)^2-(cosx/2)^2]/[(sinx/2)*(cosx/2)]
=-cosx/(1/2sinx)
=-2cosx/sinx
原式=2(cosx)^2/(-2cosx/sinx)
=-cosx*sinx=-1/2sin(2x)