F'(x)=x'*(x-1)(x-2)(x-3)…(x-1000)+x(x-1)'*(x-2)(x-3)…(x-1000)+…+x(x-1)(x-2)(x-3)…(x-1000)'
=(x-1)(x-2)(x-3)…(x-1000)+x(x-2)(x-3)…(x-1000)+…+x(x-1)(x-2)(x-3)…(x-999)
除了第一项,后面都有x,所以x=0时都等于0
所以F'(0)=(-1)(-2)(-3)…(-1000)=1000!