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Transport phenomenion problem

  1. Oct 11, 2011 #1
    i got this from my notws and i jsut can't seem to figure out how the second step is obtained? can someone explain to me where the where r's in the numerator came from on the second line when dividing by the volume of the shell?

    Attached Files:

  2. jcsd
  3. Oct 12, 2011 #2
    That is the original r from the previous equation.
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