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Using magnification formula for lenses


Lara Croft is testing concave lenses to make good eyepieces for her binoculars before the next mission.
In one experiment, Lara places a 14 cm long pin to the left of a concave lens. She obtains a 2.0 cm long virtual image at a distance of 3.0 cm to the left of the lens.
Find the object distance, u, of the pin.
Note: Let's write the answer in the Cartesian sign convention.
  • 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