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Math Help - Inversely proportional

  1. #1
    Member helloying's Avatar
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    Inversely proportional

    A spot in the room is illuminated by a lamp, dcm away.The intensity I of the light spot is inversely proportional to the square of d. At certain distance d cm, the intensity is 60 units. What is the intensity when the distance is doubled?
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  2. #2
    Flow Master
    mr fantastic's Avatar
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    Quote Originally Posted by helloying View Post
    A spot in the room is illuminated by a lamp, dcm away.The intensity I of the light spot is inversely proportional to the square of d. At certain distance d cm, the intensity is 60 units. What is the intensity when the distance is doubled?
    I = \frac{k}{d^2}.

    60 = \frac{k}{d_1^2} .... (1)

    I_2 = \frac{k}{(2d_1)^2} = \frac{k}{4 d_1^2} = \frac{1}{4} \, \frac{k}{d_1^2} .... (2)

    Substitute (1) into (2).
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