For a concave mirror, focal length (f) is negative, f<0

When an object s placed on the left side of the mirror, the object distance (u) is negative, u<0

 Using lens formula

1/ v  - 1/ u = 1/f

1/v = 1/f -1/u  ......(i)

The object lies between f and 2f

2f<u<f

1/2f>1/u>1/f

1/f -1/2f<1/f-1/u<0 .......(ii)

Using equation (i) we get

1/2f< 1/v<0                if, 1/v is negative

or, 1/2f<1/v

or, 2f>v

-v>-2f

Therefore, the image lies beyond 2f