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B Calculus of variation. Minimum surface

  1. Jul 2, 2017 #1
    so df/dy' is yy'/ √(1+y'^2)

    then we are supposed to do
    y' . [ yy'/ √(1+y'^2) ] - y√(1+y'^2)

    how does this bring equation 2 in the image ?
     

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  2. jcsd
  3. Jul 2, 2017 #2
    1499009110143.jpeg

    imaged rotated :)
     
  4. Jul 2, 2017 #3
    Multiply ##f = y \sqrt{1+y'^2}## by ##1 = \frac{\sqrt{1+y'^2}}{\sqrt{1+y'^2}}## then subtract from ##y' f_y'##.
     
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