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Homework Help: Z = xy, dz/dx = delta z/ delta x, no idea why

  1. Dec 3, 2015 #1
    1. The problem statement, all variables and given/known data
    Ok this isn't really homework just something I came across and am confused by. I came across a function that looks similar to z = xy and I found that delta_z / delta_x = dz/dx which is really weird to me. This was really strange to me so I checked to see if z = x²y would also be the same and it wasn't which I expected. I don't know how to explain why z = xy has this sort of property where delta_z / delta_x = dz/dx. Sorry if this is a sort of weird question.

    2. Relevant equations
    For example let's say z = xy, x = 5, y = 2, z = 10. We increase x to 6 then z = 12.
    delta_z / delta_x = dz/dx = y => delta_z = y*delta_x = 2*1 = 2. No idea why this is true though it should be like a linear approximation.

    3. The attempt at a solution
    So I was thinking it's true in the same way that for y=mx+b, delta_y/delta_x = dy/dx, now I don't know what to call these type of functions.
  2. jcsd
  3. Dec 3, 2015 #2


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    Hello jrm, :welcome:

    With this function, you can see that if you keep y constant, then z = constant * x, the equation of a straight line.
  4. Dec 3, 2015 #3


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    First, what do you mean by "similar to z= xy"? Was the function z= xy or not? Second, since you are talking about "delta z/delta x" and "dz/dt", what happens to y? If you are treating y as a constant, then z is a linear function of x so of course you get that property. And you call those functions "linear" functions!
  5. Dec 3, 2015 #4
    Ah so this function is linear? I looked online and it showed linear multivariable functions have the form f(x1,x2,x3,...) = a1x1 + a2x2 + a3x3 + ... So I didn't want to call this type of function linear since it does not have this form. By similar I mean it looks like z = (1+y)(A+Bx). Sorry I made a mistake in my first post. It should have been y = 10, and increases to y = 12, not z. I guess a follow up question would be anything of the form f(x1,x2,x3,...) = x1*x2*x3... would have this behavior if you kept all other variables constant when taking a derivative with respect to one variable and it would be called linear?

    Thanks for the responses!
  6. Dec 3, 2015 #5


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    You found the correct criterion for linearity. That doesn't exclude the possibility that the intersection of the 3D graph of your function with any plane y = constant (or, alternativly, x = constant) is a straight line.
    Note that for functions of more than one variable we use partial derivatives: Function ##\ z = f(x,y)\ ## has partial derivatives ##\ \partial z\over \partial x\ ## and ##\ \partial z\over \partial y\ ##.

    ##\ \partial z\over \partial x\ ## is a function of x and y obtained by taking the the usual derivative wrt x while keeping y constant. In your ##z = xy## example ##\ {\partial z\over \partial x} = y\ ##.

    And in your z = (1+y)(A+Bx) ##\ {\partial z\over \partial x} = B(1+y)\ ##, which still is a straight line -- hence the ##\ {\Delta z\over \Delta x} = {\partial z\over \partial x}\ ##.


    Picture shows intersections with y = -2 is a straight line. So is intersection with x = 2 -- and any plane with x = constant or y = constant.

  7. Dec 3, 2015 #6
    Ah, that clears things up for me. Thanks!
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