Creating a diagram from polynomial

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I defined a polynomial
P = [1 -2 4]
and I want to draw the shape of polynomial for the range between -10 to 10 with in the logarithmic diagram (only X axis). How can I do it?

Accepted Answer

Walter Roberson
Walter Roberson on 30 Aug 2022
Edited: Walter Roberson on 30 Aug 2022
P = [1 -2 4];
x = linspace(-10, 10, 500);
y = polyval(P, x);
lx = log10(x);
rlx = real(lx);
ilx = imag(lx);
plot3(rlx, ilx, y)
xlabel('real(log(x))'); ylabel('imag(log(x))'); zlabel('polynomial value')
You need to use a 3D plot for this because you want a log axes on a range that includes negative values, so you need some way to represent the component of the log of the negative numbers.

More Answers (1)

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 30 Aug 2022
There are a few different ways to get this exercise done.
Compute the values of the polynomial P = [1 -2 4] at x = -10:dx:10 with some increment step size (dx) and then plot x vs. y using semilogx(). The code starts with these:
P = [1 -2 4];
dx = ...
x= -10:dx:10;
y = P(1)*x.^2+P(2)*x+P(3);
If you have a difficulty to undertand how to plot the computed values, use help.
>> help semilogx
Moreover, your plotted data will contain the positive values of x and the negative side will be ignored due to a log scale.


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