Plotting the third and firth order polynomial of a function?

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Matt Amador 2017-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/362426-plotting-the-third-and-firth-order-polynomial-of-a-function

评论： Image Analyst 2017-10-21

Hello, I have some trouble with my code. The objective of it is to fit the following data using a third and fifth order polynomial and plot the fits over the range 0<=t<=10

Data: >>t = 0:10;
      >> y = [0 0.5104 0.3345 0.0315 -0.1024 -0.0787 ...
               -0.0139 0.0198 0.0181 1.0046 -0.0037];

All of this data comes from the function: y(t) = e^(-0.5*t) * sin(t)

Here is my code so far:

clc
clear
t = 0:10;
y = exp(-0.5.*t)*sin(t);
p1 = polyfit(t,y,3);
p2 = polyfit(t,y,5);
plot(p1)
hold on
plot(p2)
grid on

I'm getting an error saying that the inner matrix dimensions must agree. Can anyone help out?

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Jan 2017-10-20

Please do not write a shortened rephrased error message, but a complete copy.

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回答（1 个）

Jan 2017-10-20

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/362426-plotting-the-third-and-firth-order-polynomial-of-a-function#answer_286930

在 MATLAB Online 中打开

t = 0:10;
y = exp(-0.5 .* t) .* sin(t);  % Elementwise .* instead of matrix multiplication *

Read the documentation of polyfit. You will find out, that the coefficients of the polynomial are replied. It is not meaningful to plot them. But polyval let you obtain the curves again. Use a finer grid then, e.g. tt = 0:0.1:10.

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Jan 2017-10-21

@Image Analyst: You mean "polyval", not "polyfit".

@Matt Amador: I assumed, that this is a homework and did not post a working solution. If this assumption was wrong: Sorry, I did not want to conceal the solution, but to let you the chance to solve it by your own.

Image Analyst 2017-10-21

Jan, right - polyval (right in the code, wrong in the description - sorry).

Matt, if someone posts homework, the expectation is that they label it as homework so people don't give a complete solution, lest they get caught for plagiarism and get in trouble. However, I gave you partial code based on your original code (plus a few other fancy things) so it might be okay. But if it's homework, the safest thing to do is to label it as homework. We wouldn't want you to get in trouble.

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