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Whoops got it now, didn't carry out my substitutions far enough.

[tex]

z = x^2 + 2y^2

[/tex]

[tex]

x = rcos(\theta)

[/tex]

[tex]

y = rsin(\theta)

[/tex]

Find [tex](\partial z/\partial x)[/tex] (theta is constant)

dz = 2xdx + 4ydy

dx = cos[tex](\theta)[/tex]dr - rsin[tex](\theta)[/tex]d[tex]\theta[/tex]

dy = sin[tex](\theta)[/tex]dr + rcos[tex](\theta)[/tex]d[tex]\theta[/tex]

Unfortunately I'm not really quite sure where to go from here, I know that

[tex](\frac{ \partial z } { \partial x} )[/tex] is 2x when y is constant. But how to factor in theta being constant?

I suppose I could reduce

dx to dx = cos[tex](\theta)[/tex]dr

and dy = sin[tex](\theta)[/tex]dr

## Homework Statement

[tex]

z = x^2 + 2y^2

[/tex]

[tex]

x = rcos(\theta)

[/tex]

[tex]

y = rsin(\theta)

[/tex]

## Homework Equations

## The Attempt at a Solution

Find [tex](\partial z/\partial x)[/tex] (theta is constant)

dz = 2xdx + 4ydy

dx = cos[tex](\theta)[/tex]dr - rsin[tex](\theta)[/tex]d[tex]\theta[/tex]

dy = sin[tex](\theta)[/tex]dr + rcos[tex](\theta)[/tex]d[tex]\theta[/tex]

Unfortunately I'm not really quite sure where to go from here, I know that

[tex](\frac{ \partial z } { \partial x} )[/tex] is 2x when y is constant. But how to factor in theta being constant?

I suppose I could reduce

dx to dx = cos[tex](\theta)[/tex]dr

and dy = sin[tex](\theta)[/tex]dr

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