Stuck on math in deriving an optics equation

[Puchinita5]

Homework Statement

Hi. For a project I have to derive this formula for bright fringes in optics, but I'm stuck on this simple math part (or maybe its not so simple)...ive attached a picture of where i don't understand...it might be considered a simple math problem so I'm sorry if this is too trivial a question for physics forums :)

Homework Equations

The Attempt at a Solution

 

[ehild]

(1+a)^1/2 can be approximated by 1+a/2 if a<<1.

For example:

a=0.02: 1.02^1/2=1.00995...~1.01,

a=0.002: 1.002^1/2=1.0009995...~1.001

ehild