Let \(\displaystyle y(x) = \sqrt{x}\). Expand this in a Taylor series (with x small compareed to a):
\(\displaystyle y(a - x) \approx y(a) + \left . \dfrac{dy}{dx} \right | _{x = a}\cdot (a - x)\)

Let \(\displaystyle y(x) = \sqrt{x}\). Expand this in a Taylor series (with x small compareed to a):
\(\displaystyle y(a - x) \approx y(a) + \left . \dfrac{dy}{dx} \right | _{x = a}\cdot (a - x)\)