# Mathematica cant graph

1. Apr 4, 2005

### Pengwuino

Ok i got this problem in this file. Can someone help?

If you dont wanan get the file the problem is basically this.

Haave an f(x,y) equation. Find both partial derivatives. One graphs correctly, other partial derivative is just a straight line on y=0. Anyone know whats causing this?

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2. Apr 4, 2005

### saltydog

1. First calculate the partials manually first and then plot them.

2. Verify the Mathematica code:

fpartx[x_,y_]=D[f(x,y),x]
fparty[x_,y_]=D[f(x,y),y]

is actually calculating the partials.

3. Use Plot3D and manually insert the partial directly first, you know, like:

Plot3D[x^2+y+2 x y,{x,a,b},{y,a,b}] or whatever the partial is.

4. Use Plot3D[fpartx[x,y],{x,a,b},{y,a,b}]

gotta work.

3. Apr 4, 2005

### Pengwuino

Plot3D works but the implicitplot doesnt, check it out.

Whats going on :)

Last edited: Apr 4, 2005
4. Apr 4, 2005

### saltydog

So the partial with respect to y is:

$$f_y=4xy-64y$$

When you equate that to zero you get x=16. So ImplicitPlot graphs a vertical line at x=16. Extend the PlotRange to see it.

5. Apr 4, 2005

### Pengwuino

Ok ill check it out later, sounds like that might be the problem though!