Two thin biconvex lenses have focal lengths $f_1$ and $f_2$. A third thin biconcave lens has a focal length of $f_3$. If the two biconvex lenses are in contact, the total power of the lenses is $P_1$. If the first convex lens is in contact with the third lens, the total power is $P_2$. If the second lens is in contact with the third lens, the total power is $P_3$. Which of the following is correct?
$P_1 = \frac{f_1 f_2}{f_1 - f_2}, P_2 = \frac{f_1 f_3}{f_3 - f_1} \text{ and } P_3 = \frac{f_2 f_3}{f_3 - f_2}$
$P_1 = \frac{f_1 - f_2}{f_1 f_2}, P_2 = \frac{f_3 - f_1}{f_3 + f_1} \text{ and } P_3 = \frac{f_3 - f_2}{f_3 + f_2}$
$P_1 = \frac{f_1 - f_2}{f_1 f_2}, P_2 = \frac{f_3 - f_1}{f_3 f_1} \text{ and } P_3 = \frac{f_3 - f_2}{f_3 f_2}$
$P_1 = \frac{f_1 + f_2}{f_1 f_2}, P_2 = \frac{f_3 - f_1}{f_1 f_3} \text{ and } P_3 = \frac{f_3 - f_2}{f_1 f_3}$
