# Jacobian of a composed function+chain rule

• Jul 26th 2009, 09:22 PM
arbolis
Jacobian of a composed function+chain rule
Consider the following functions : $\displaystyle f\begin{pmatrix} u \\ v \end{pmatrix}=\begin{pmatrix} u+v \\ u-v \\ u^2-v^2 \end{pmatrix}$ and $\displaystyle F(x,y,z)=x^2+y^2+z^2=w$.
1)Find the Jacobian of $\displaystyle F \circ f$ in $\displaystyle f\begin{pmatrix} a \\ b \end{pmatrix}$.
2)Find $\displaystyle \frac{\partial w}{\partial u}$ and $\displaystyle \frac{\partial w}{\partial v}$.

My attempt :
1) $\displaystyle (F \circ f)(a,b)=(a+b)^2+(a-b)^2+(a^2-b^2)^2$. And the Jacobian is $\displaystyle \left ( 2a(2+2a^2-b^2) \hspace{1 cm} 2b(2+2b^2-a^2) \right )$. Is it right?

2)$\displaystyle \frac{\partial w}{\partial u}=\frac{\partial w}{\partial x}\cdot \frac{\partial x}{\partial u}+\frac{\partial w}{\partial y}\cdot \frac{\partial y}{\partial u}+\frac{\partial w}{\partial z}\cdot \frac{\partial z}{\partial u}$. But I'm stuck on how to find $\displaystyle \frac{\partial x}{\partial u}$ for example. Any idea?
• Jul 26th 2009, 11:29 PM
Moo
Hello,

For question 2), what I don't see is how w and u can be related, as they're just dummy variables (that is to say : their name is not important) (Surprised)
• Jul 27th 2009, 09:37 AM
arbolis
Quote:

Originally Posted by Moo
Hello,

For question 2), what I don't see is how w and u can be related, as they're just dummy variables (that is to say : their name is not important) (Surprised)

Unfortunately the question is as I posted here. I guess I have to use the Jacobian I calculated in part 1), but I don't see how.