Domain sketch for multiple variables function

How do I sketch the domain of the function f(x,y) = ln(9 - x^2 - 9y^2)?

This is for my exercise so I really need some help. Thanks in advanced!

Answer 1

[Walter Roberson]

For example

expr = @(x,y) -x.^2 - 2.*x.*y - 7.*y.^2 + 11.*y + 1;
[X,Y] = meshgrid(linspace(-5,5,200));
eZ = expr(X,Y);
eZ(eZ<=0) = nan;
Z = log(eZ);
surf(X, Y, Z, 'edgecolor', 'none')
xlabel('x'); ylabel('y')

[r,c] = find(~isnan(eZ),1)

r = 124

c = 30

[X(r,c), Y(r,c), eZ(r,c), Z(r,c)]

ans = 1×4

-3.5427 1.1809 0.0446 -3.1101