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Homework Help: Partial differentiation confused

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data

    If x = yz and y = 2sin(y+z), find dx/dy and d^2x/dy^2

    2. Relevant equations

    3. The attempt at a solution

    I am beyond confused at how to even start this one, the problem is not like any of the examples in my book.

    I know x = ydz + zdy but dont know how to deal with z and dz?

    Thanks for the help
  2. jcsd
  3. Dec 16, 2009 #2
    Find dz in terms of dy from the second equation and plug it into the first: dx=ydz+zdy.
  4. Dec 16, 2009 #3
    Is it correct to use inverse trig function for that?
  5. Dec 16, 2009 #4


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    Science Advisor

    I can see no reason to. The second equation is y= 2sin(y+z) so dy= 2cos(y+z)(dy+ dz). From dx= zdy+ ydz, (NOT the "x= zdy+ ydz" you give. I assume that was a typo.), we get ydz= dx- zdy so dz= (1/y)dx- (z/y)dy. Putting that into the second equation, dy= 2cos(y+z)(dy+ (1/y)dx- (z/y)dy)= 2cos(y+z)(1- (z/y)dy+ (2/y)cos(y+z)dx. Solve that for dx and divide both sides by dy.
  6. Dec 16, 2009 #5
    You can do anything you want that doesn't violate the laws of math!
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