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Thin lens formula - virtual object

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An object O is placed in front of a concave mirror. The image created is 40 cm from the mirror as shown.
Next, a lens is placed at the point P, 34 cm from the mirror. The image is now formed 39 cm from the mirror.
Find the focal length of the convex lens.
Note: Use proper sign convention while reporting your answer.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi