Paul's Online Notes
Paul's Online Notes
Home / Calculus III / Partial Derivatives / Partial Derivatives
Show Mobile Notice Show All Notes Hide All Notes
Mobile Notice
You appear to be on a device with a "narrow" screen width (i.e. you are probably on a mobile phone). Due to the nature of the mathematics on this site it is best views in landscape mode. If your device is not in landscape mode many of the equations will run off the side of your device (should be able to scroll to see them) and some of the menu items will be cut off due to the narrow screen width.

Section 13.2 : Partial Derivatives

9. Find \(\frac{{\partial z}}{{\partial x}}\) and \(\frac{{\partial z}}{{\partial y}}\) for the following function.

\[{x^2}\sin \left( {{y^3}} \right) + x{{\bf{e}}^{3z}} - \cos \left( {{z^2}} \right) = 3y - 6z + 8\]

Show All Steps Hide All Steps

Start Solution

Okay, we are basically being asked to do implicit differentiation here and recall that we are assuming that \(z\) is in fact \(z\left( {x,y} \right)\) when we do our derivative work.

Let’s get \(\frac{{\partial z}}{{\partial x}}\) first and that requires us to differentiate with respect to \(x\). Just recall that any product involving \(x\) and \(z\) will require the product rule because we’re assuming that \(z\) is a function of \(x\). Also recall to properly chain rule any derivative of \(z\) to pick up the \(\frac{{\partial z}}{{\partial x}}\) when differentiating the “inside” function.

Differentiating the equation with respect to \(x\) gives,

\[2x\sin \left( {{y^3}} \right) + {{\bf{e}}^{3z}} + 3\frac{{\partial z}}{{\partial x}}x{{\bf{e}}^{3z}} + 2z\frac{{\partial z}}{{\partial x}}\sin \left( {{z^2}} \right) = - 6\frac{{\partial z}}{{\partial x}}\]

Solving for \(\frac{{\partial z}}{{\partial x}}\) gives,

\[2x\sin \left( {{y^3}} \right) + {{\bf{e}}^{3z}} = \left( { - 6 - 3x{{\bf{e}}^{3z}} - 2z\sin \left( {{z^2}} \right)} \right)\frac{{\partial z}}{{\partial x}}\hspace{0.5in} \to \hspace{0.5in} \require{bbox} \bbox[2pt,border:1px solid black]{{\frac{{\partial z}}{{\partial x}} = \frac{{2x\sin \left( {{y^3}} \right) + {{\bf{e}}^{3z}}}}{{ - 6 - 3x{{\bf{e}}^{3z}} - 2z\sin \left( {{z^2}} \right)}}}}\] Show Step 2

Now we get to do it all over again except this time we’ll differentiate with respect to \(y\) in order to find \(\frac{{\partial z}}{{\partial y}}\). So, differentiating gives,

\[3{y^2}{x^2}\cos \left( {{y^3}} \right) + 3\frac{{\partial z}}{{\partial y}}x{{\bf{e}}^{3z}} + 2z\frac{{\partial z}}{{\partial y}}sin\left( {{z^2}} \right) = 3 - 6\frac{{\partial z}}{{\partial y}}\]

Solving for \(\frac{{\partial z}}{{\partial y}}\) gives,

\[3{y^2}{x^2}\cos \left( {{y^3}} \right) - 3 = \left( { - 6 - 3x{{\bf{e}}^{3z}} - 2zsin\left( {{z^2}} \right)} \right)\frac{{\partial z}}{{\partial y}}\hspace{0.5in} \to \hspace{0.5in} \require{bbox} \bbox[2pt,border:1px solid black]{{\frac{{\partial z}}{{\partial y}} = \frac{{3{y^2}{x^2}\cos \left( {{y^3}} \right) - 3}}{{ - 6 - 3x{{\bf{e}}^{3z}} - 2zsin\left( {{z^2}} \right)}}}}\]