f(x)=x^(1/3),
f(x)≈f(x0)+f`(x0)f(x-x0)
得
990^(1/3)
=(1000-10)^(1/3)
=10x(1-10/1000)^(1/3)
≈10x[1-1/3x（10/1000）]
≈9.96667