If f1 = 12 cm, f2 = – 18 cm find the maximum focal length of the combination?

36 cm 34 cm 32 cm 30 cm

36 cm

$\frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} = \frac{1}{12} + \frac{1}{-18} = \frac{3-2}{36}$ $\Rightarrow F = 36cm$