An object is at a distance $x_1$ from the focus of a concave mirror,and its image is formed at a distance $x_2$ from the focus. The focal length of the mirror is:

  • A
    $x_1 x_2$
  • B
    $\sqrt{x_1 x_2}$
  • C
    $(x_1 + x_2) / 2$
  • D
    $\sqrt{x_1 / x_2}$

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