Plotting equations in 3D

I have the equation:

x^2 + y^2 + z^2 = 1

How do I tell Matlab to plot this in the 3D plane? This is just one equation, but I will need to plot several in the 3D plane.

[Adam]

doc plot3

[Marco Froelich]

The problem is not the plot command, rather the actual code for the formula. When I enter it, I get an error message saying:

The expression to the left of the equal sign is not a valid target for an assignment.

[Marco Froelich]

Another error comes up with another formula: x = y + z

undefined function or variable y

[Adam]

I think you would need the Symbolic Toolbox to do this kind of thing. I only glanced at the equation initially and didn't register the '= 1' at the end of it which turns it into something that base Matlab doesn't cater for.

I've never worked with equation solving in Matlab myself so hopefully someone else can provide more information on usage of the Symbolic Toolbox or whatever other functionality is needed for this.

[Marco Froelich]

No but I am not trying to solve the equation, only plot its graph. But fair enough, thank you

Answer 1

[Ari]

Your equation x^2 + y^2 + z^2 = 1 resembles a surface and can be plotted with the fsurf command in MATLAB which will need your function handle as an argument. You will need to rewrite the function as z expressed in terms of x and y as follows.

z = @(x,y) sqrt(x.^2 + y.^2 - 1); % function handle to anonymous function
fsurf(z)

You can find other methods to do the same from the documentation here .

For information on function handles see here .

For plotting multiple 3D surfaces on the same graph you can use the hold command as below. The hold command will plot subsequent plots in the same figure.

z1 = @(x,y) sqrt(x.^2 + y.^2 - 1); 
z2 = @(x,y) sqrt(x.^2 + 2*y.^2 - 5); 
fsurf(z1)
hold on
fsurf(z2)

[Marco Froelich]

Im sorry, but I just tried this and it gave me a weird graph I will show you. The equation above is supposed to give me sphere.

[Marco Froelich]

I found it. Input function, with the command fimplicit3

[Sakib Mahmud]

This equation will give half of a sphere. The rest half will come from -sqrt...

Answer 2

[HARSH MEHTA]

x^2 + y^3 + z^4 = f(x,y,z)

and X^2(y^3)(z^4) = f(x,y,z) how can i polt this both differently