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