Plotting graph using roots

I have calculated the roots of ? = ? 3 + 5.5? 2 + 3.5? − 10 (p=[1 5.5 3.5 -10]; >> x=roots(p) x = -4.0000 -2.5000 1.0000) How do I plot this on a single graph (The curve y for −5 ≤ ? ≤ 3.). Any help will be appreciated:)

Answer 1

[ME]

The roots you have calculated are the x values where y = 0. Therefore to plot these on a graph you can use:

p = [1 5.5 3.5 -10];
x = roots(p);
xaxis = -5:0.1:3;
y= xaxis.^3 + 5.5.*xaxis.^2 + 3.5.*xaxis - 10;
plot(xaxis,y,'-',x,zeros(size(x)),'o');

This shows the curve of the function over your specified interval and then plots circles at the roots you have obtained to highlight their location on the plot.

[Mark Davies]

Thank you this is perfect :) I was having trouble with how to type this into Matlab.

[ME]

No problem. I'm glad it was useful to you.

[Steven Lord]

One suggestion: since you already have the coefficients of the polynomial stored as the vector p, you could simplify the code that evaluates y by using polyval to evaluate the polynomial for your xaxis values. That way there's only one place where the polynomial is defined, and you'll avoid potential confusion if you change p but forget to change the line where you evaluate the polynomial and so see mismatched results.

[Mark Davies]

Thanks for the info :)