1/√99.9
近似值微分公式 f(x0+△x)≈f(x0)+f'(x0)△x
设f(x)=x^(-1/2)(x>0)
f(99.9)≈f(100)+f'(100)*(99.9-100)
=1/10+[(-1/2)/100^(3/2)](-0.1)
=1/10+0.05/1000
=0.10005