Question

A smaller, inverted image is located a distance of 5.0 cm from a concave mirror with the object distance of 25.0 cm. Determine the focal length.

Answer 1

1/v + 1/u = 1/f

1/5.0cm +1/25.0cm = 1/f

so,

we got, focla length = 6/25 = - 4.1 cm

Answer 2

$u = 25cm \\ v = 5cm \\ sign \: covention \\ u = - 25 \\ v = - 5 \\ \\ \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\ \frac{1}{f } = \frac{1}{ - 25} + \frac{1}{ - 5 } \\ \: \: \: \: \: = \frac{ - 1}{25} + \frac{ - 5}{25} \\ \: \: \: \: \: = \frac{ - 6}{25} \\ \frac{1}{f} = \frac{ - 6}{25} \\ f = \frac{ - 25}{6} \\ f = - 4.1 cm$