由f‘(a)=lim(x→0)　[f(a+x)-f(a)] / x
f‘(0)=lim(x→0) x(x-1)(x-2)(x-3)(x-4)(x-5)/x
=lim(x→0) (x-1)(x-2)(x-3)(x-4)(x-5)
=-1×2×3×4×5
=-120