How to plot cubic equations ?

Hello all,

I'm new to matlab and am stuck trying to plot the function x*y*(x^2+y^2)>=47.3.

can someone explain/show how both the equality and inequality can be plotted.

Thanks.

Answer 1

[Star Strider]

Here is one approach:

f = @(x,y) x.*y.*(x.^2+y.^2) - 47.3;

figure
fih = fimplicit(f, [-10 10  -10 10]);
xv = fih.XData;
yv = fih.YData;
v0 = ones(size(xv));
Lpos = xv>=0;
xpos = xv(Lpos);
ypos = yv(Lpos);
xneg = xv(~Lpos);
yneg = yv(~Lpos);
xneg = xneg(~isnan(xneg));
yneg = yneg(~isnan(xneg));
hold on
% fimplicit(f2, [-10 10  -10 10])
yl = ylim;
xl = xlim;
patch([xpos fliplr(xpos)], [ones(size(ypos))*max(yl) fliplr(ypos) ], 'r')
patch([fliplr(xneg) xneg], [ones(size(yneg))*min(yl) (yneg) ], 'r')
hold off
grid

This is not a straightforward problem, so it could be difficult, especially since the NaN values could be difficult to find and elimiinate, and they are not obvious.

The filled areas are those that conform to the stated inequality.

I evaluated them from -10 to 10 with respect to both variables. Change those to get different results.

[Amith Mallya]

Thanks worked really well, it was more complicated then i had thought.

[Star Strider]

As always, my pleasure!

I agree. It was more complicated than I originally thought it would be, as well.