Use the mirror equation to deduce that :
an object placed between f and 2f of a concave mirror produce a real image beyond 2f

Explanation

For a concave mirror, the mirror formula is given by
`(1)/(v) = (1)/(f) - (1)/(u)`
`therefore` The object lies between f and 2f ,
`2f gt u gt f or, (1)/(2f) lt (1)/(u) lt (1)/(f)`
`or, (1)/(2f) - (1)/(f) lt (1)/(u) - (1)/(f) lt (1)/(f) - (1)/(f)`
`or, -(1)/(2f) lt -((1)/(f)-(1)/(u)) lt 0 or, -(1)/(2f) lt - (1)/(v) lt 0`
`therefore " " 0 gt v gt 2f`
Since v is-ve, the image is real and lies beyond 2f.