Starting 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_(0)`.