Error with plotting function

I want to plot z = (1.5-x1+x1*x2)^2 +(2.25-x1+x1*(x1^2))^2+(2.625-x1+x1*(x2)^3)^2 with -5<x1<5 and -5<x2<5.

I tried:

x1 = -5:5;

x2 = -5:5;

z = (1.5-x1+x1*x2).^2 +(2.25-x1+x1*(x1.^2)).^2+(2.625-x1+x1*(x2).^3).^2;

plot(x1,x2,z);

What am I doing wrong? It gives error: Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To perform elementwise multiplication, use '.*'.

Answer 1

[Star Strider]

You will need to use element-wise operators with vector arguments. See Array vs. Matrix Operations for details.

Use surf for the plot.

The fsurf function can also be used plot the resulting surface —

z = @(x1,x2) (1.5-x1+x1.*x2).^2 +(2.25-x1+x1.*(x1.^2))^2+(2.625-x1+x1.*(x2).^3).^2;
x1 = -5:5;
x2 = -5:5;
[X1,X2] = ndgrid(x1,x2);

figure
surf(X1, X2, z(X1,X2))
grid
xlabel('X_1')
ylabel('X_2')
zlabel('z')

syms x1 x2
z = (1.5-x1+x1*x2)^2 +(2.25-x1+x1*(x1^2))^2+(2.625-x1+x1*(x2)^3)^2;

figure
grid
fsurf(z, [-5  5  -5  5])
xlabel('X_1')
ylabel('X_2')
zlabel('z')

EDIT — Corrected typographical errors.

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