In a concave mirrorr experiment, an object is placed at a distance `x_(1)` from the focus and the image is formed at a distance `x_(2)` from the focus. The focus length of the mirrorr would be
A. `x_(1)x_(2)`
B. `sqrt(x_(1)x_(2))`
C. `(x_(1)+x_(2))/(2)`
D. `sqrt(x_(1)/x_(2))`