Question

size of a virtual image in a concave mirror of focal length 20 cm is thrice the size of object. what is the distance between the object and the concave mirror

Answer 1

focal length is negative in concave mirror.
i hope u will help

Answer 2

The focal length of concave mirror is negative.

For real image,

1/v + 1/u =1/f

By substituting we get,

1/-3u + 1/u = - 1/20 (v= -3u)

=> -1+3/3u  = -1/20

=> 2/3u = -1/20

=> 2 × -20 = 3u

=> u = -40/3 cm