Getting incorrect plot – curve does not cross the x axis correctly

Hello,

I'm trying to plot a quadratic equation with highlighted roots and I am facing a problem – my curve does not cross x axis correctly.

For example:

y = 2x^2 - 4x, => roots are x1 = 2, x2 = 0 and I get this graph:

So the blue curve should cross the red points but does not. I dont know why.

My code for plotting this is like this:

x = [min(x1, x2)-5:0.1:max(x1, x2)+5];
y = (koef_a*x).^2 + koef_b*x + koef_c;
plot(x, y, 'b'), grid on, xlabel('x'), ylabel('y');
ax = gca; % Get handles to axis.
ax.YAxisLocation = 'origin';
ax.XAxisLocation = 'origin';
hold on;
plot(x1, 0, 'o', 'MarkerSize', 5, 'MarkerFaceColor', 'r', 'MarkerEdgeColor', 'r');
plot(x2, 0, 'o', 'MarkerSize', 5, 'MarkerFaceColor', 'r', 'MarkerEdgeColor', 'r');
hold off;

I would be very grateful for any help. Thanks

Answer 1

[Star Strider]

The roots are [0 2], so use those with the poly function:

rts = [0 2];
coefs = poly(rts)

coefs = 1×3

1 -2 0

figure
fplot(@(x) x.^2 - 2.*x, [-5 5])
hold on
scatter(rts, [0 0], 'r', 'filled')
hold off
grid
set(gca,'XTick',-5:5)