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Over- and under-estimation of Riemann sums

AP.CALC:
LIM‑5 (EU)
,
LIM‑5.A (LO)
,
LIM‑5.A.1 (EK)
,
LIM‑5.A.2 (EK)
,
LIM‑5.A.3 (EK)
,
LIM‑5.A.4 (EK)
Riemann sums are approximations of area, so usually they aren't equal to the exact area. Sometimes they are larger than the exact area (this is called overestimation) and sometimes they are smaller (this is called underestimation).

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  • orange juice squid orange style avatar for user Sarahkondo
    Would the average of the right and left Riemann sums get you the actual area under the curve?
    (15 votes)
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    • spunky sam blue style avatar for user Joseph Barshay
      Interesting question! Not exactly. The average of the right and left Riemann sums of a function actually gives you the same result as if you had used a trapezoidal approximation (instead of rectangular). This approximation is closer to the actual area of the function though!
      (21 votes)
  • purple pi purple style avatar for user Kevin Liu
    Is there a general rule when RRAM is greater than LRAM? Is there also a rule to determine which (RRAM MRAM or LRAM) is the most accurate depending on the situation?
    (3 votes)
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    • ohnoes default style avatar for user Joseph Sivits
      RRAM will always be greater than LRAM when the function is increasing, and LRAM will always be greater than RRAM when the function is decreasing.

      LRAM/RRAM in my opinion is horrible way of estimation, but for AP they will tell you when to use LRAM/RRAM in the problem, and later MRAM as well.
      (12 votes)
  • male robot hal style avatar for user AwesomeBillionareNerd
    If you take an average of a right Riemann Sum and Left Riemann Sum do you get the exact area
    (2 votes)
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  • sneak peak purple style avatar for user Vector Inc.
    At and onward, a rectangle seems to form between the right-edge rectangle and left-edge rectangle. The function also seems to cut through them, very close to halfway cut. Could this affect how we use Riemann sums?
    (2 votes)
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    • female robot grace style avatar for user loumast17
      Not exactly. Though for many equations if you average out the left and right rectangle sums you usually get closer to the true area under a graph. Of course there are some graph shapes where this is untrue, or the size of the rectangles makes it untrue.

      With using a riemann sum though it doesn't matter if you do left or right in the end, since eventually riemann sums as you to divide the graph into infinitely many rectangles, or other shapes. This makes it so it doesn't matter where you start, the infinitely many rectangles will fit perfectly under the graph.

      I'm not sureif you knew that yet or where this video is on the playlist, so apologies if that is confusing.
      (2 votes)
  • blobby green style avatar for user Nina Jauhari
    Hi,

    For right hand rule:
    For example:
    My interval is from 1-9, the function is 1/x and lastly I need to take four intervals. Would I take my first point as f(1) and my second as f(3)? If yes, why? And if not, why?

    Thank you!
    (2 votes)
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  • duskpin seed style avatar for user 侯佳葳
    Is it possible to calculate the trapezoid instead of the rectangles ?
    (1 vote)
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  • blobby green style avatar for user Patrick Couto
    what if the function is decreasing and in the third/fourth quadrant? because then it would be upside down?
    (2 votes)
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    • hopper cool style avatar for user Iron Programming
      Howdy Patrick,

      Good question! But the key here is that we usually don't talk about "negative area". Instead, we would say that it estimates the area under the x-axis. Thus we are still having less area.

      Note that sometimes we want to calculate the net area, where we subtract the area below the x-axis from the area above the x-axis. In this case, you would be right that the left Riemann sum would be underestimating the amount that should be subtracted, and thus is overestimate the overall sum (provided that there is more area below the x-axis than above the x-axis: otherwise the underestimation from above the x-axis might cancel out the overestimation from below the x-axis).

      Yeah... a bit of a tongue twister there, but I hope you get the point.
      (1 vote)
  • duskpin ultimate style avatar for user mlijah
    why not take a triangle and subtract it from the left Riemann sum? The triangle would be ((left side function value - right side function value)*change of x) / 2. That would be a slight under approximation but would be far more accurate.
    (1 vote)
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  • aqualine ultimate style avatar for user BigBrain
    Can you use the extra rectangle, the difference between the left and right and half that and then add the answer onto the underestimate, will that be the closest you can get if you don't change the number of rectangles in total?
    (1 vote)
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  • piceratops ultimate style avatar for user Rithick
    couldn't you use rectangles then triangles to get even more accurate or add the left and right reimman and divide by two to get way closer
    (1 vote)
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Video transcript

- [Narrator] Consider the left and right Riemann sums that would approximate the area under y is equal to g of x between x equals two and x equals eight. We want to approximate this light blue area right here. Are the approximations over estimations or underestimations? So, let's just think about each of them. Let's consider the left and right Riemann sums. First the left. I'm just gonna write left for short but I'm talking about the left Riemann sum. They don't tell us how many subdivisions to make for our approximation so that's up to us to decide. Let's say we went with three subdivisions. Let's say we want to make them equal. They don't have to be, but let's say we do. The first one would go from two to four, the next one would go from four to six, and the next one would go from six to eight. If we do a left Riemann sum, you use the left side of each of these subdivisions in order to find the height. You evaluate the function at the left end of each of those subdivisions for the height of our approximating rectangles. We would use g of two to set the height of our first approximating rectangle, just like that. Then we would use g of four for the next rectangle. We would be right over there. Then you would use g of six to represent the height of our third and our final rectangle, right over there. Now, when it's drawn out like this, it's pretty clear that our left Riemann sum is going to be an overestimation. Why do we know that? Because these rectangles, the area that they're trying to approximate is contained in the rectangles. And these rectangles have this surplus area so they're always going to be larger than the areas that they're trying to approximate. And in general, if you have a function that's decreasing over the interval that we care about right over here, strictly decreasing the entire time. If you use the left edge of each subdivision to approximate, you're going to have an overestimate. Because the left edge, the value of the function there, is going to be higher than the value of the function at any of the point in the subdivision. That's why for decreasing function, the left Riemann sum is going to be an overestimation. Now let's think about the right Riemann sum and you might already guess that's going to be the opposite but let's visualize that. Let's just go with the same three subdivisions. But now let's use the right side of each of these subdivisions to define the height. For this first rectangle the height is going to be defined by g of four. That's right over there. And then for the second one it's going to be g of six. That is right over there. For the third one it's going to be g of eight. Let me shade these in to make it clear which rectangles we're talking about. This would be the right Riemann sum to approximate the area. It's very clear here that this is going to be an underestimate. Underestimate because we see in each of these intervals, the right Riemann sum or the rectangle that we're using for the right Riemann sum is a subset of the area that its trying to estimate. We're not able to, it doesn't capture this extra area right over there. And once again, that is because this is a strictly decreasing function. So if you use the right end point of any one of these or the right side of any of these subdivisions in order to define the height, that right value of g is going to be the lowest value of g in that subdivision so it's going to be a lower height than what you could say is even the average height of the value of the function over that interval. So you're going to have an underestimate in this situation. Now, if your function was strictly increasing, then these two things would be swapped around and of course, there are many functions that are neither strictly increasing or decreasing and then it would depend on the function. Sometimes even, it would depend on the type of subdivisions you choose to decide whether you have an overestimate or an underestimate.