Plotting the exp(-x) using the power series expansion and for loop

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Tb 2020-10-29

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https://ww2.mathworks.cn/matlabcentral/answers/630448-plotting-the-exp-x-using-the-power-series-expansion-and-for-loop

评论： Tb 2020-10-29

采纳的回答： Ameer Hamza

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Hello,

I am trying to program the function exp(-x) using the power series and for loop and plotting it against the actual exp(-x) to check what the error is, however, for some reason the program does not show the correct graph for the power series expansion. My code is below:

x1 = 0:0.1:5;
y = exp(-x1);
figure
plot(x,y,'k') % plots the true function e^-x in black
hold on;
ex1 = ones(1,51);
ex2= 0;
 for n = 1:5
     ex1 = ex1 + ((-ex1).^n).*((x1.^n)/(factorial(n)))
 end
 
 plot(x1,ex1,'r');
 

The issue is with the expression for ex1, but I can't figure out why it isn't working as I am sure that I have used the correct formulae. Thanks.

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采纳的回答

Ameer Hamza 2020-10-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/630448-plotting-the-exp-x-using-the-power-series-expansion-and-for-loop#answer_528118

在 MATLAB Online 中打开

The series expansion is wrong. Check the following code

x1 = 0:0.1:5;
y = exp(-x1);
figure
plot(x1,y,'k') % plots the true function e^-x in black
hold on;
ex1 = zeros(size(x1));
for n = 0:5
    ex1 = ex1 + (-1).^n.*(x1.^n)/(factorial(n));
end
plot(x1,ex1,'r');