∫（1/sinx）dx =∫1/[2sin（1/2）xcos（1/2）x]dx
=∫d(x/2)/[tan(x/2)（cos^2(x/2)]
=∫dtan(x/2)/tan(x/2)=Ln/tan(x/2)/+C
=Ln/cscx－cotx/＋C