Find two positions of an object, placed in front of a concave mirror of focal length 15cm, so that the image formed is 3 times the size of the object.

Explanation

f = 15 cm
m =3
i) `m=(-upsilon)/(u)=(f)/(f-u)`
`3=(15)/(15-u)`
`45-3u=15`
3u = 30
u = 10 cm.
ii) `m=(f)/(u-f)`
`3=(15)/(u-15)`
`3u-45=15`
3u = 60
u = 20 cm.