How is this radical transformed?

Question

Wave equation derivation

In the link shown above, there is a derivation of the wave equation. At line (1), the author says that he divides equation by $\Delta x$ and takes the limit as this quantity is approaching 0. How does the equation go from p(x)$\sqrt{\Delta x^2 + \Delta u^2}$$\frac{\partial^2u}{\partial x^2}$ to p(x)$\sqrt{1 + (\frac{\partial u}{\partial x})^2}$$\frac{\partial^2u}{\partial x^2}$ on the left side? I don't understand that at all.