Two long solenoids of radii r_1 and r_2(> r_1) and number of turns per unit length n_1 and n_2 respectively are co-axially wrapped one over the…

Two long solenoids of radii $\displaystyle \mathrm{r}_{1}$ and $\displaystyle \mathrm{r}_{2}\left(>\mathrm{r}_{1}\right)$ and number of turns per unit length $\displaystyle \mathrm{n}_{1}$ and $\displaystyle \mathrm{n}_{2}$ respectively are co-axially wrapped one over the other. The ratio of self-inductance of inner solenoid to their mutual inductance is-