How to calculate partial area under a graph?

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friet
friet 2016-11-29
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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Star Strider
Star Strider 2016-11-29
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
  2 个评论
Namira
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
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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