For some reason, I am not getting plots when I put in this code for this question.

The question states: insert the plots obtained for the following three cases in the figure below.

In the title of the plot replace “Roots of Polynomial” with your “Last_Name, FirstName”

a. a=1, b=3, c=4

b. a=1, b=4, c=4

c. a=1, b=3, c=2

My code says:

% Enter the real coefficients of a quadratic polynomial
disp('Enter the coeffs of the quadratic: a x^2 + b x + c')
a = input('Enter the coeff. of x^2 term: a = 1');
b = input('Enter the coeff. of x^1 term: b =3 ');
c = input('Enter the coeff. of x^0 term: c =4 ');
% Compute the Discriminant: Delta = b^2 - 4ac
Delta = b^2 - 4*a*c;
% Solve for the roots of the quadratic polynomial
if Delta < 0        % Case of real and distinct roots
    x1 = (-b + sqrt(Delta)) / 2*a;
    x2 = (-b - sqrt(Delta)) / 2*a;
    fprintf('  Real and distinct roots: x1 = %4.2f  x2 = %4.2f\n', x1, x2)
elseif Delta == 0   % Case of real repeated roots
    x1 = -b / (2*a);
    x2 = x1;
    fprintf('  Real and repeated roots: x1 = %4.2f  x2 = %4.2f\n', x1, x2)
else                % Case of complex roots
    x1 = (-b + sqrt(Delta)) / (2*a);
    x2 = (-b - sqrt(Delta)) / (2*a);
    fprintf('  Imag roots: x1 = %g + j%g and x2 = %g + j%g \n',...
        real(x1), imag(x1), real(x2), imag(x2));
end
plot = [num2str(a), 'x^2 + ', num2str(b), 'x + ',num2str(c)];
% Plot the two roots
poly(real(x1), imag(x1),'rx', real(x2), imag(x2),'bo', 'MarkerSize',16, 'LineWidth',5)
title(['Roots of polynomial: f(x) = ', poly, ' = 0'], 'FontSize', 16)
xlabel('Real Axis');
ylabel('Imag Axis')
legend(['x_1=',num2str(x1)],['x_2=',num2str(x2)])
grid

Basically, my problem is that I am not getting any graphs and plots from my code. What should I do differently?

[Walter Roberson]

plot = [num2str(a), 'x^2 + ', num2str(b), 'x + ',num2str(c)];

That creates a variable named plot that is a character vector. After you execute that line, you will not be able to call plot() as a function until that plot variable is cleared or goes out of scope.

Answer 1

[Torsten]

if Delta > 0        % Case of real and distinct roots
    x1 = (-b + sqrt(Delta)) / (2*a);
    x2 = (-b - sqrt(Delta)) / (2*a);
    fprintf('  Real and distinct roots: x1 = %4.2f  x2 = %4.2f\n', x1, x2)

instead of

if Delta < 0        % Case of real and distinct roots
    x1 = (-b + sqrt(Delta)) / 2*a;
    x2 = (-b - sqrt(Delta)) / 2*a;
    fprintf('  Real and distinct roots: x1 = %4.2f  x2 = %4.2f\n', x1, x2)
    

And I don't know what you expect from the command

    poly(real(x1), imag(x1),'rx', real(x2), imag(x2),'bo', 'MarkerSize',16, 'LineWidth',5)

Maybe you mean

    plot(real(x1), imag(x1),'rx', real(x2), imag(x2),'bo', 'MarkerSize',16, 'LineWidth',5)

But this will only plot 2 points. A graph of the parabola together with the roots/root/non-existing real roots would be helpful.