The function you coded:
FUN = @(x) = x(1).^2+x(2).^2;
will only take the first 2 elements of ‘x’ (whatever it is), and will return a scalar.
You need to restate it as:
FUN = @(x1,x2) x1.^2+x2.^2;
for example, to produce a surface:
[X1,X2] = meshgrid(-1:0.5:1);
figure
surf(X1, X2, FUN(X1,X2))
grid on
.
