x=3sect,dx=3sect*tantdt
∫√（x的平方-9）dx/x
=∫3tant*3sect*tant*costdt
=∫9(tant)^2 dt
=∫(9(sect)^2-9)dt
=9tant-9t+c
=9√（x的平方-9）/x-9*arccos(3/x)+c