定義:
y'=lim dx->0 [y(x+dx)-y(x)]/dx
=lim dx->0 [根號((x+dx)^2+1)-根號(x^2+1)]/dx
分子有理化,上下同乘[根號((x+dx)^2+1)+根號(x^2+1)]
注意分子是(a-b)(a+b)=a^2-b^2,根號抵消
=lim dx->0 [((x+dx)^2+1)-(x^2+1)]/[dx(根號((x+dx)^2+1)+根號(x^2+1))]
=lim dx->0 (2x*dx+dx^2)/[dx(根號((x+dx)^2+1)+根號(x^2+1))]
=lim dx->0 (2x+dx)/[(根號((x+dx)^2+1)+根號(x^2+1))]
然后把dx=0代入,得到
y'=2x/[2根號(x^2+1)]=x/根號(x^2+1)