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Image formed by multiple lenses

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Problem

Between an object and a screen are two lenses as shown below.
The object is located 18, space, start text, c, m, end text from the first lens of focal length f, start subscript, 1, end subscript, equals, plus, 15, space, start text, c, m, end text. The second lens is located 15, space, start text, c, m, end text from the screen and has a focal length f, start subscript, 2, end subscript, equals, plus, 20, space, start text, c, m, end text.
What should be the distance between the two lenses to obtain a sharp image on the screen?
d, equals
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3, slash, 5
  • a simplified improper fraction, like 7, slash, 4
  • a mixed number, like 1, space, 3, slash, 4
  • an exact decimal, like 0, point, 75
  • a multiple of pi, like 12, space, start text, p, i, end text or 2, slash, 3, space, start text, p, i, end text
start text, c, m, end text
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