# Convex lens of 10 cm focal length is combined with a concave lens of 6 cm focal length. Find focal length of combination.

$(a)\;-20\;cm \\ (b)\;-5\;cm \\ (c)\;-15\;cm \\ (d)\;-10\;cm$

$f_1= 10\; cm$
$f_2= -6 \;cm$
use formula,
$\large\frac{1}{F}=\frac{1}{f_1} +\frac{1}{f_2}$
$\qquad = \large\frac{1}{10}-\frac{1}{6}=\frac{-1}{15}$
$F=-15\; cm$
Hence c is the correct answer.