dx/√(x^2＋a^2)^3
设x=atant dx=a(sect)^2dt
∫dx/√(x^2＋a^2)^3
=∫a(sect)^2dt/(asect)^3
=(1/a^2)∫costdt
=(1/a^2)sint+C
=(1/a^2)(x/‍√(x^2＋a^2))+C