taylor series and electrical potential

Question

a) Find four non-zero terms of the Taylor series of the function
√a a2 + x2

 

b) The electric potential, V , at a distance R from the centre of a charged disk with radius a and constant charge density σ, is given by
V = 2πσ(√R2 + a2 − R) . Show that , for large R,
V ≈ πa2σ .

Answer 1

How does electric potential change as you move closer and closer to a negative point charge?