Find an expression for the electric field(Mastering Physics)

[Eric Fossum]

I just wanted to post a solution for anyone interested. Any notes on how to solve would be nice for others.

1. Homework Statement

Mastering Physics (3rd edition) Problem 26.40

The figure (Figure 1) shows a thin rod of length L with total charge Q.

Part A) Find an expression for the electric field strength on the axis of the rod at distance r from the center.

Part B) Verify that your expression has the expected behavior if r≫L.
Express your answer in terms of variables Q, rand constants π, ε0.

Part C) Evaluate E at r = 3.6cm if L = 5.0 cm and 2.2nC .
Express your answer to two significant figures and include the appropriate units.

3. The solution
Part A)

Part B)

Part C)

 

[Simon Bridge]

It is usually more help to post the method for obtaining solutions, the reasoning etc., rather than the bare answer as you have done here.
Why are those the correct answers? How would someone figure those out for themselves?