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Homework Help: Chain Rule

  1. Sep 12, 2016 #1
    1. The problem statement, all variables and given/known data
    Suppose $$z=x^2 sin(y), x=5t^2-5s^2, y=4st$$
    Use the chain rule to find $$\frac{\partial z}{\partial s} \text{ and } \frac{\partial z}{\partial t}$$

    2. Relevant equations
    $$\frac{\partial z}{\partial s} = \frac{\partial z}{\partial x} \frac{\partial x}{\partial s} + \frac{\partial z}{\partial y} \frac{\partial y}{\partial s}$$

    3. The attempt at a solution
    $$\frac{\partial z}{\partial x} = 2x\sin(y)$$
    $$\frac{\partial x}{\partial s} = -10s$$
    $$\frac{\partial z}{\partial y} = x^2\cos(y)$$
    $$\frac{\partial y}{\partial s} = 4t$$
    With those I then substitute the values into the equation, and I came up with $$\frac{\partial z}{\partial s}=2x\sin(y)(-10s)+x^2\cos(y)(4t)=-20(5t^2-5s^2)(s)\sin(4st)+4(5t^2-5s^2)(t)\cos(4st)$$
    Where am I going wrong? Also, I am still learning Latex, so I apologize for the crude display.
  2. jcsd
  3. Sep 12, 2016 #2

    Ray Vickson

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    Science Advisor
    Homework Helper

    In the last term you should have ##(5t^2-5s^2)^2##, not ##(5t^2-5s^2)##.
  4. Sep 12, 2016 #3
    Thanks! Never fails I overlook something simple like that. Other than that, does it appear that I am going about it correctly?
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