lim(x->0) [x^2.f(x) -2f(x^3)]/x^3 (0/0)
分子，分母分别求导
=lim(x->0) [2x.f(x)+x^2.f'(x) -6x^2.f'(x^3)]/(3x^2)
=lim(x->0) [2f(x)+xf'(x) -6xf'(x^3)]/(3x)
=lim(x->0) 2f(x)/(3x) + lim(x->0)[f'(x) -6f'(x^3)]/3
=(2/3){ lim(x->0) [f(x)-f(0)]/x } + [f'(0) -6f'(0)]/3
=(2/3)f'(0) - (5/3)f'(0)
=-f'(0)
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