令x=3sect
上式=∫3tant/3sectd3sect=3∫tan^2tdt
=3∫sec^2tdt-3t
=3tant-3t+C
tant=√(x^2-9)]/3
t=arctan(√(x^2-9)]/3)
上式=√(x^2-9)]+3arctan(√(x^2-9)]/3)+C