How to calculate partial area under a graph?

Question

friet 2016-11-29

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https://ww2.mathworks.cn/matlabcentral/answers/314577-how-to-calculate-partial-area-under-a-graph

评论： Star Strider 2017-5-10

Hello,

I am trying to find the area as a part of a graph( Red area in the picture below). I am asking here a simplified question of my complext problem. The area is definitely 0.5. However, when I am using "trapz" function, It is giving me 4.5.

x=[0 1 2 3 4 5 10 15 20];
y=[0 -1 -2 -3 -4 -5 0 5 0];
plot(x,y)
area=trapz(4:5)

Thanks, Frite

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KSSV 2016-11-29

编辑：Walter Roberson 2016-11-30

You have asked the same question....and it got answered too.... http://www.mathworks.com/matlabcentral/answers/314470-how-can-i-calculate-the-area-under-a-graph-shaded-area

friet 2016-11-29

编辑：John Kelly 2016-12-12

I ask another simplified version of the previous question.

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Answer 1

Star Strider 2016-11-29

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https://ww2.mathworks.cn/matlabcentral/answers/314577-how-to-calculate-partial-area-under-a-graph#answer_245224

在 MATLAB Online 中打开

You are not coding the integration correctly to integrate the area you want.

Explore this and experiment with it:

x=[0 1 2 3 4 5 10 15 20];
y=[0 -1 -2 -3 -4 -5 0 5 0];
yi = interp1(x, y, [4:5]);
shaded_area = trapz([4:5],yi+5)
plot(x,y)
hold on
patch([4 5 5 4], [yi -5 -5], 'r')
hold off
shaded_area =
   500.0000e-003

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Namira 2017-5-10

I was looking your code. Why did you add 5 with yi in line 4? I can't understand it. Sorry I am not good at Matlab coding.

Star Strider 2017-5-10

I added 5 because the area to be integrated goes from -5 to the line. The trapz function would otherwise integrate from 0 to the line. The offset ‘resets’ the 0 reference to -5.

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