Two thin lenses of focal lengths $f_1 \: and \: f_2$ are in contact and coxial. The power of the combination is :

Question

$\begin {array} {1 1} (1)\;\large\frac{f_1+f_2}{f_1f_2} & \quad (2)\;\sqrt{\large\frac{f_1}{f_2}} \\ (3)\;\sqrt{\large\frac{f_2}{f_1}} & \quad (4)\;\large\frac{f_1+f_2}{2} \end {array}$