Where Does the Self Inductance Formula Come From?

[nabliat]

http://i27.tinypic.com/23r02mg.jpg

I CANT UNDERSTAND FROM WHERE THEY GET THE FORMULA
IN THE SOLUTION

 

[gabbagabbahey]

A is the cross sectional area of the solenoid/telephone cord, and the magnetic field inside the solenoid is B=\mu_0 NI/l so the flux through one loop in the solenoid is BA=\mu_0 N IA/l so the total flux is \Phi= \mu_0 N^2 I A/l and the inductance is just total flux divided by current.

 

[nabliat]

thanks