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Thread: Calculate an area under graph?

  1. #1
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    Hi everyone, I'm trying to find way to calculate area under graph, in other words integral.Does anyone know how to do it? My graph is buld of sinle poits.
    Thanks in advance.
    arguing with an idiot, make sure he isn't doing the same

  2. #2
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    How accurate do you need the area to be? Are the points evenly spaced? If so a simple approximation is just the sum of the y values times the interval between them.

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  3. #3
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    Thanks Derk & thomach. I took a look at the links. In general i know how ingergral works.But in my case i have function that has exponential behaviour. The values are measured not equal interval of times. Well, i don't understand by myself what i've wrote here. I will explain you my whole issue so it will be clear what i want to do:
    I building a battery for my old laptop (which consists of 10 sequently connected 1.2v rechargable batteries) I connected everything. The main problem is to check if they are charged properly . In other words i have to determine when to stop the charge in order to not overcharge them. So here what i do. I put empy batt. pack and starting to charge it, every couple of minutes i measure charge current and time and adding into excell. at the beginning current is big, and getting smaller with time with exponential behaviour. The area under graph - is battery capacity (mAh). It's a pity i cannot attack file to show you. So i than have to make some graph approximatian from those points that i entered. The only thing i found is "Add trendline" where i can choose a polynom or othe func typle (log, etc) that will be approximation to my curve. Excell does it perfectly. I.E i right-click on my ponts, choose Add trendline, choose appropriate polynom, and see a line that repeats my points very accurate (i don't need extreme precision) But i don't see a formula of this curve????. That's the problem, if i only could see a forumula i could calculate integral by myself, for example (5x^3-6x^2-x+25x^(-1)) etc.

    If you got tired reading all this mess, BOttom line question:
    I have some graph (curve) How can i know it's formulla f(x) ??? at least approximately?
    Thanks in advance for your help
    arguing with an idiot, make sure he isn't doing the same

  4. #4
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    I can't offer a direct answer, but you might have a look at the chart Area sheet from the "Average Line" tutorial that Andy Pope provides via this link:

    http://www.andypope.info/charts/averageline.htm
    Last edited by Dave Hawley; December 23rd, 2009 at 06:51.
    Best Regards,
    Tom
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    Please help Oz share knowledge among all users by posting your questions in a public forum rather than using a Private Message.

  5. #5
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    If you are happy with the trend line and just want the equation, Excel will give it to you! When you are at the add trendline dialog box, click on the Options tab and select display equation. Tha's it. Then integrate the equation.

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  6. #6
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    Thanks you guys, "display equation" in trendline dialog- is exactly what i wanted. How culd i missed it??.
    Thanks again.
    arguing with an idiot, make sure he isn't doing the same

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