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## Homework Statement

A light bulb is placed 10.7 cm in front of a concave mirror. When the concave mirror is replaced by a plane mirror in the same location, the bulb's image moves 4.00 cm closer to the mirror. Calculate the focal length of the concave mirror.

*I am given: do: 10.7*

and 4.00 is Di

and 4.00 is Di

## Homework Equations

Mirror equation: 1/di+1/do=1/f

## The Attempt at a Solution

I know I have to use the mirror equation so I did. However, I also added 10.7 and 4.00 to get a new di.

1/10.7+1/14.7=1/f

Eventually, I get 6.19 which is wrong! Anyone would like to help me see what is wrong?[/B]