A concave mirror produces a two times magnified virtual image of an object kept 10 cm in front of it. Calculate the focal length of the mirror.
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For mirrors:
\[
m=-\frac{v}{u}
\]
and
\[
\frac{1}{f}
=
\frac{1}{u}
+
\frac{1}{v}.
\]
A virtual image formed by a concave mirror is always erect and has positive magnification.