Strating from the expression for the energy `W=(1)/(2)LI^(2)`, stored in a solenoid of self-inductance L to build up the current I, obtain the expression for the magnetic energy in terms of the magnetic field B, area A and length l, of the solenoid having n number of turns per unit length. Hence show that the energy density is given by `B^(2)//2mu_(o)`.

Explanation

Use the relation `B=(mu_(0)NI)/(2r)` for `B_(1)` and `B_(2)` then find its resultant.