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Homework Help: Concave mirror positioning help

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data
    A concave shaving mirror has a radius of curvature of 40 cm. It is positioned so that the (upright) image of a man's face is 3.30 times the size of the face. How far is the mirror from the face?

    2. Relevant equations
    M = Si/S 1/S + 1/Si = 2/R

    3. The attempt at a solution
    I have 2 equations with 2 unknowns. I isolated Si in the first one and substituted this expression into equation 2 to find S. I get an answer of 26.1cm.

    The computer keeps telling me I'm wrong though. Could someone check my math or tell me if my equation setup is correct?
  2. jcsd
  3. Jan 29, 2009 #2
    oops never mind i got it... 13.94cm
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