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Can someone explain this summation definition to me?

  1. Nov 18, 2014 #1

    Consider the function f(x)=4√x, 1≤x≤16. Using the above definition, determine which of the following expressions represents the area under the graph of f as a limit.

    I knew the correct answer was [tex]\sum \frac{15}{n} (4√x+\frac{15i}{n}) [/tex]

    I figured out most of this, but the only thing I don't get is how you figure out is basically since you start the sum from i=1 to n, that you have to shift the sum up. If i=0, then I think there would be no "1+" term, right?
    But let's say it was.... i =5 to n. I have no idea why, or where you would put the added terms in order for the sum to work. I thought that it would be outside of the function 4√x but inside of the summnation, because, well, you're just adding values, right?
    I don't see or understand intuitively why 1+ would go inside of 4√x.

    Similarly, I got this right but didn't understand the idea.

    Consider the function [tex]f(x) = \frac{ln(x)}{x}[/tex],3≤x≤10. Using the above definition, determine which of the following expressions represents the area under the graph of f as a limit.

    Of course, the answer was Δx and [tex]\frac{ln 3+\frac{7i}{n}}{3+\frac{7i}{n}}[/tex], but just like above, wasn't sure why the [tex]\frac{ln3}{3}[/tex] went there. I thought if anything, it should be an added term, not mixed up with the main fraction.
    Last edited by a moderator: Nov 19, 2014
  2. jcsd
  3. Nov 19, 2014 #2


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    The area is

    $$\lim_{n\rightarrow\infty}\sum_{i=1}^n \dfrac{b-a}{n}\mathrm{f}\left(a+i\dfrac{b-a}{n}\right)$$

    The function values are determined by dividing the interval into equal parts. a+i(b-a)/n are the points that that divide the interval evenly.
  4. Nov 20, 2014 #3
    Thanks, I understand that, but just about the notation- Do you mean to say f(x)? And the X being all the stuff in parentheses?

    But sorry, I have another confusion- if a is on the inside of those parentheses, isn't A the starting value? Then why, for example is the answer for that lnx/x one not the A value from, 3≤x≤10, which would make it 3, right? Instead of ln3?
  5. Nov 20, 2014 #4


    Staff: Mentor

    I'm pretty sure he didn't mean f(x) or he would have written that. To understand what that notation means, try it out with a simple function like f(x) = x2 and an interval [0, 2].

    See what you get with, say, n = 4. The summation will have 4 terms.
    a is the left endpoint of the interval.
    This doesn't make much sense, so I don't know what you're asking.
  6. Nov 24, 2014 #5


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    It would have been clearer with parentheses. You should have written
    $$\frac{\ln \left(3 + \frac{7i}n\right)}{3 + \frac{7i}n}$$ which I hope you recognize as ##f\left(3 + \frac{7i}n\right)## when ##f(x) = \frac{\ln x}{x}##.
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