由题可知,f(x)=ax²+o(x²)
u=x-f(x)/f'(x)
lim u/x=lim [1-f(x) / xf'(x)]
而lim f(x) / xf'(x) =lim f'(x) / [f'(x) + xf''(x)] = lim f''(x) / [f''(x) + f''(x) + xf'''(x) ] = f''(0) / (2f''(0) + 0) = 1/2,所以lim u/x=1/2
且lim f(u)/f(x)=lim [au²+o(u²)]/[ax²+o(x²)]=lim u²/x²=1/4
所以原式=[lim f(u)/f(x)]/(lim u/x)=1/2