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Area and element of area

  1. Oct 8, 2015 #1
    If A = x y (if the area of the paralelepid A is equal to edge x multiplied by edge y), so, dA is equal dx y + x dy. See:

    But this is so much incovenient! The convenient would be dA = dx dy.

    Let's see now d²A...

    d²A = d dA = d²x y + dx dy + dx dy + x d²y

    Now dx dy appears! But, is not a convenient expression, because d²x y and x d²y appears too in the equation!

    How to solve this problem? We want that A = x y and dA = dx dy. Do you understand this conceptual problem of defition?
  2. jcsd
  3. Oct 8, 2015 #2
    It would also be very convenient that every number is rational.
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