Two coaxial long solenoids of equal length have current, `i_(1), i_(2)`, number of turns per unit

Two coaxial long solenoids of equal length have current, `i_(1), i_(2)`, number of turns per unit length `n_(1), n_(2)` and radius `r_(1), r_(2)` respectively. If `n_(1)i_(1)=n_(2)i_(2)` and the two solenoids carry current in opposite sence, the magnetic energy stored per until length is `[r_(2)gt r_(1)]`