An object is placed at a distance `x_1` from the principal focus of a lens and its real image is formed at a distance `x_2` from the another principal focus. The focal length of the lens is

With a concave mirrorr, an object is placed at a distance x_(1) from the principal focus, on the principal axis. The image is formed at a distance x_(2) from the principal focus. The focal length of the mirrorr is

With a concave mirror, an object is placed at a distance x_(1) from the principal focus, on the principal axis. The image is formed at a distance x_(2) from the principal focus. The focal length of the mirror is

With a concave mirror, an object is placed at a distance x_(1) from the princiipal focus, on the principal axis. The image is formed at a distance x_(2) from the principal focus. The focal length of the mirror is

With a concave mirror, an object is placed at a distance 9cm from the principal focus, on the principal axis. The image is formed at a distance 16cm from the principal focus. The focal length of the mirror is

An object is placed at a distance X_1 from the focus of a concave mirror, Its real image is formed at a distance X_2 from the focus, Show that the focal length of the mirror is sqrt(x_1x_2)

An object placed at a distance of a 9cm from the first principal focus of a convex lens produces a real image at a distance of 25cm. from its second principal focus. then the focal length of the lens is :