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# How to plot cubic equations ?

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Amith Mallya on 3 Mar 2021
Commented: Star Strider on 4 Mar 2021
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.
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### Accepted Answer

Star Strider on 3 Mar 2021
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.
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Star Strider on 4 Mar 2021
As always, my pleasure!
I agree. It was more complicated than I originally thought it would be, as well.

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