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Clarification on the output of partial derivatives

  1. Apr 17, 2012 #1
    1. In the Khan academy video I watched on partial derivatives, I understand absolutely everything except for the last 20 seconds which confused me.

    http://www.youtube.com/watch?v=1CMDS4-PKKQ

    Using the formula:

    Z = x² + xy + y²

    @z/@x = 2x +y

    x=0.2, y=0.3

    2(.2) + .3 = .7

    What I know this means: At an x value of 0.2 and a y constant of 0.3, the slope of the tangent line will be 0.7.

    At the end of the Khan video though, he stated:

    "So everytime X increases, 1, Z will increase by 0.7"

    Did I miss something or is this a mistake? What is in the video is a curve that is constantly changing so the slope will always be different. The only way to say that every increase in X will result in 0.7 is if we are dealing with a planar surface correct?
     
  2. jcsd
  3. Apr 17, 2012 #2
    Yes, your interpretation is correct. He misspoke and should have said z increases approximately 0.7 units from its value at (0.2, 0.3) if x increases from 0.2 to 1.2 using a linear approximation from that point. The linear approximation deviates from the curve; the only way to get the actual increase is to evaluate the nonlinear function z at each point.
     
  4. Apr 18, 2012 #3
    Thanks. I was frustrated thinking that I didn't understand some crucial general concept but now am good.
     
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