$\mathrm{v}=\mathrm{f}_{1} \lambda_{1}=\mathrm{f}_{2} \lambda_{2}$
now Beats $=\mathrm{f}_{1}-\mathrm{f}_{2}$
$\mathrm{n}=\frac{\mathrm{v}}{\lambda_{1}}-\frac{\mathrm{v}}{\lambda_{2}} \mathrm{v}=\frac{\mathrm{n} \lambda_{1} \lambda_{2}}{\lambda_{2}-\lambda_{1}}$
