令x=siny
原式=∫1/(sinycosy)*cosydy
=∫1/[2cos^2 (y/2)]/tan(y/2)dy
=∫d(tany/2)/tan(y/2)
=ln|tan(y/2)|+C
=ln|(1-cosy)/siny|+C
=ln|[1-√(1-x^2)]/x|+C