# Implicit differentiation help

• Aug 12th 2012, 01:25 PM
Implicit differentiation help
Hi,

I need some help with some implicit differentiation.

I have a system where ${\color{red}x}$ in a choice variable that someone gets to pick. For each choice of ${\color{red}x}$ the following non-linear system of equations gets solved for ${\color{red}y}$ and ${\color{red}z}$.
$F(x,y,z)&=&0\\G(x,y,z)&=&0$
and spits out a unique $(x,y(x),z(x))$. This $(x,y(x),z(x))$ is then used to evaluate the expression $H(x,y,z)$. Now I am writing ${\color{red}y(x)}$ and ${\color{red}z(x)}$ but ${\color{red}y}$ and ${\color{red}z}$ cannot be expressed in terms of elementary functions of ${\color{red}x}$ since the above system of equations cannot be solved analytically.

The idea is to find an ${\color{red}x}$ where $\frac{d H(x,y(x),z(x))}{dx} =0$. I don't actually want to solve this problem right away in the sense that I don't need the ${\color{red}x}$ that solves the first order condition, rather I need the expression of the first order condition itself which I need to plug in somewhere else. That's why I am not using Lagrange multipliers here.

Any help understanding the math here at a conceptual level and making progress on this problem will be greatly appreciated.

Thanks
Anshu