Question:

In a concave mirror, an object is placed at a distance d1 from the focus and the image is formed at a distance d2 from the focus. Then the focal length of the mirror is:

d1d2

(d1+d2)/2

√d1/d2

√d1d2

Let f be the magnitude of the focal length of the mirror.
Using mirror equation , 1/v + 1/u = -1/f
=> 1/-(d1+f) + 1/-(d2+f) = -1/f
=> f = √d1d2