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Area of a ring

  1. Oct 22, 2015 #1
    How was the equation for area of a ring derived?
     

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  3. Oct 22, 2015 #2
    I can derive the area as the difference of the areas of two circles. ##= \pi(R+dR)^2 - \pi R^2##
    But I don't think this ##\pi (R+dR)^2 - \pi R^2## is equal to ## (2/pi R)(dR)## given in the beginning.
     
  4. Oct 22, 2015 #3

    PeroK

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    No, but it's an approximation for small ##dR##.
     
  5. Oct 22, 2015 #4

    mathman

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    When setting up integrals, the differential is assumed to be vanishing small, so that terms proportional to the powers of the differential can be neglected.
     
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