Riemann Sums and Notation

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AP Calculus AB › Riemann Sums and Notation

Questions 1 - 10
1

Which definite integral is represented by $\displaystyle \sum_{i=0}^{9} \frac{1}{1+(1+0.1i)^2}(0.1)$?

$\displaystyle \int_{0}^{1} \frac{1}{1+x^2},dx$

$\displaystyle \int_{1}^{1.9} \frac{1}{1+x^2},dx$

$\displaystyle \int_{0}^{9} \frac{1}{1+(1+0.1x)^2},dx$

$\displaystyle \int_{1}^{2} \frac{1}{1+x^2},dx$

$\displaystyle \int_{1}^{2} \frac{1}{1+(1+0.1x)^2},dx$

Explanation

This problem requires translating $\sum_{i=0}^{9} \frac{1}{1+(1+0.1i)^2}(0.1)$ into integral notation. The sum follows $\sum f(x_i)\Delta x$ where $f(x_i) = \frac{1}{1+(1+0.1i)^2}$ and $\Delta x = 0.1$. When $i=0$, we get $x_0 = 1+0.1(0) = 1$, and when $i=9$, we get $x_9 = 1+0.1(9) = 1.9$. Since this is a left Riemann sum (starting at $i=0$), the integral runs from $x_0 = 1$ to $x_{10} = 1+0.1(10) = 2$. The function is $f(x) = \frac{1}{1+x^2}$ because when $x = 1+0.1i$, we have $\frac{1}{1+(1+0.1i)^2} = \frac{1}{1+x^2}$. A mistake would be to write $\frac{1}{1+(1+0.1x)^2}$ as the integrand, which incorrectly substitutes the index for the variable. The key insight is that $1+0.1i$ represents the $x$-value itself, so $(1+0.1i)^2 = x^2$.

2

Which integral matches $\sum_{k=1}^{n}\left(3\frac{k}{n}+1\right)\frac{3}{n}$?

$\displaystyle \int_{0}^{1}(3x+1),dx$

$\displaystyle \int_{1}^{4}(x+1),dx$

$\displaystyle \int_{0}^{1}(x+1),dx$

$\displaystyle \int_{0}^{3}(3x+1),dx$

$\displaystyle \int_{0}^{3}(x+1),dx$

Explanation

This question tests your ability to translate a Riemann sum into its corresponding definite integral notation. The Riemann sum uses Δx = 3/n, indicating an interval width of 3. The argument 3k/n + 1 increases from approximately 1 to 4 as k goes from 1 to n, but the choice maps it to x + 1 over 0 to 3 via substitution. The integrand x + 1 corresponds to the linear expression in the sum after adjusting limits. A tempting distractor is choice B, which fails because using 3x + 1 over 0 to 3 triples the slope, resulting in a larger integral value that doesn't match the sum. A transferable translation strategy is to identify Δx as (b - a)/n, determine a and b from the range of the argument as the index varies from 1 to n, and set the integrand to match the function of that argument.

3

Which definite integral is represented by $\sum_{i=1}^{n}\sin!\left(\frac{\pi i}{2n}\right)\frac{\pi}{2n}$?

$\displaystyle \int_{0}^{\pi/2}\sin(2x),dx$

$\displaystyle \int_{0}^{\pi}\sin(x),dx$

$\displaystyle \int_{0}^{\pi/2}\sin\left(\frac{x}{2}\right),dx$

$\displaystyle \int_{0}^{\pi/2}\sin(x),dx$

$\displaystyle \int_{0}^{1}\sin\left(\frac{\pi}{2}x\right),dx$

Explanation

This problem tests the skill of translating a Riemann sum into its corresponding definite integral. The factor $\frac{\pi}{2n}$ represents \Delta x, indicating the total interval length is \pi/2. The argument $\frac{\pi i}{2n}$ maps to x_i = i \Delta x, from near 0 to \pi/2. The function is \sin(x), with limits from 0 to \pi/2. A tempting distractor is choice D, \int_${0}^{\pi/2}$\sin(2x),dx, which fails by incorrectly doubling the argument of sine. A general strategy for translating Riemann sums is to identify \Delta x = $\frac{b-a}{n}$, the form of x_i = a + i \Delta x, and the function f(x_i), so the sum approximates \int_$a^b$ f(x) dx as n approaches infinity.

4

Which integral corresponds to $\sum_{k=1}^{n}\left(4-\frac{k}{n}\right)^3\frac{1}{n}$?

$\displaystyle \int_{1}^{4}x^3,dx$

$\displaystyle \int_{0}^{1}(4+x)^3,dx$

$\displaystyle \int_{0}^{1}(4-4x)^3,dx$

$\displaystyle \int_{0}^{1}(4-x)^3,dx$

$\displaystyle \int_{0}^{4}(1-x)^3,dx$

Explanation

This problem tests the skill of translating a Riemann sum into its corresponding definite integral. The factor $\frac{1}{n}$ represents \Delta x, indicating the total interval length is 1. The expression 4 - $\frac{k}{n}$ corresponds to sample points decreasing from near 4 to 3, with the function $(x)^3$. To match, we can set limits from 0 to 1 with integrand (4 - $x)^3$, capturing the reverse progression. A tempting distractor is choice D, \int_${1}^{4}$$x^3$,dx, which fails because it uses incorrect limits of 1-4 instead of adjusting for the sum's range. A general strategy for translating Riemann sums is to identify \Delta x = $\frac{b-a}{n}$, the form of x_i = a + i \Delta x, and the function f(x_i), so the sum approximates \int_$a^b$ f(x) dx as n approaches infinity.

5

Which definite integral matches $\sum_{i=1}^{n}e^{\frac{2i}{n}}\frac{2}{n}$?

$\displaystyle \int_{0}^{1}e^{2x},dx$

$\displaystyle \int_{0}^{2}e^{x},dx$

$\displaystyle \int_{1}^{2}e^{x},dx$

$\displaystyle \int_{0}^{4}e^{x},dx$

$\displaystyle \int_{0}^{2}e^{2x},dx$

Explanation

This problem tests the skill of translating a Riemann sum into its corresponding definite integral. The factor $\frac{2}{n}$ represents \Delta x, indicating the total interval length is 2. The exponent $\frac{2i}{n}$ maps to x_i = i \Delta x, with function $e^x$. The limits are from 0 to 2, as the sample points cover near 0 to 2. A tempting distractor is choice D, \int_${0}^{2}$$e^{2x}$,dx, which fails by incorrectly doubling the exponent instead of recognizing the scaling in \Delta x. A general strategy for translating Riemann sums is to identify \Delta x = $\frac{b-a}{n}$, the form of x_i = a + i \Delta x, and the function f(x_i), so the sum approximates \int_$a^b$ f(x) dx as n approaches infinity.

6

Which integral matches $\sum_{k=1}^{n}\left(\frac{3k}{n}\right)^2\frac{3}{n}$?

$\displaystyle \int_{0}^{3}x^2,dx$

$\displaystyle \int_{0}^{1}x^2,dx$

$\displaystyle \int_{0}^{1}(3x)^2,dx$

$\displaystyle \int_{1}^{3}x^2,dx$

$\displaystyle \int_{0}^{3}(3x)^2,dx$

Explanation

This problem tests the skill of translating a Riemann sum into its corresponding definite integral. The factor $\frac{3}{n}$ represents \Delta x, indicating the total interval length is 3. The expression $\left($\frac{3k}{n}$\right)^2$ maps to x_k = k \Delta x, with function $x^2$. The limits are from 0 to 3, as points go from near 0 to 3. A tempting distractor is choice C, \int_${0}^{3}$$(3x)^2$,dx, which fails by incorrectly scaling the integrand with 3 inside. A general strategy for translating Riemann sums is to identify \Delta x = $\frac{b-a}{n}$, the form of x_i = a + i \Delta x, and the function f(x_i), so the sum approximates \int_$a^b$ f(x) dx as n approaches infinity.

7

Which integral matches $\sum_{k=1}^{n}\left(\frac{k}{n}\right)^4\frac{1}{n}$?

$\displaystyle \int_{0}^{4}x,dx$

$\displaystyle \int_{1}^{n}\left(\frac{x}{n}\right)^4\frac{1}{n},dx$

$\displaystyle \int_{0}^{1}x^5,dx$

$\displaystyle \int_{0}^{1}(4x)^4,dx$

$\displaystyle \int_{0}^{1}x^4,dx$

Explanation

This problem tests the skill of translating a Riemann sum into its corresponding definite integral. The factor $\frac{1}{n}$ represents \Delta x, indicating the total interval length is 1. The expression $\left($\frac{k}{n}$\right)^4$ maps to x_k = k \Delta x, with function $x^4$. The limits are from 0 to 1, covering the sample points from near 0 to 1. A tempting distractor is choice E, \int_${0}^{1}$$(4x)^4$,dx, which fails by incorrectly introducing a factor of 4 in the integrand. A general strategy for translating Riemann sums is to identify \Delta x = $\frac{b-a}{n}$, the form of x_i = a + i \Delta x, and the function f(x_i), so the sum approximates \int_$a^b$ f(x) dx as n approaches infinity.

8

Which integral corresponds to $\sum_{k=1}^{n}\left(\frac{\pi k}{n}\right)\frac{\pi}{n}$?

$\displaystyle \int_{1}^{\pi}x,dx$

$\displaystyle \int_{0}^{\pi}x,dx$

$\displaystyle \int_{0}^{\pi}\pi x,dx$

$\displaystyle \int_{0}^{1}\pi x,dx$

$\displaystyle \int_{0}^{1}x,dx$

Explanation

This question tests your ability to translate a Riemann sum into its corresponding definite integral notation. The Riemann sum uses Δx = π/n, indicating an interval width of π. The argument π k/n increases from approximately 0 to π as k goes from 1 to n, mapping to the integral limits from 0 to π. The integrand x corresponds directly to the linear expression in the sum. A tempting distractor is choice B, which fails because integrating π x from 0 to 1 misses an additional π factor, resulting in a value π times smaller than the sum's limit. A transferable translation strategy is to identify Δx as (b - a)/n, determine a and b from the range of the argument as the index varies from 1 to n, and set the integrand to match the function of that argument.

9

Which definite integral matches $\sum_{i=1}^{n}\left(\frac{4i}{n}\right)^3\frac{4}{n}$?

$\displaystyle \int_{1}^{4}x^3,dx$

$\displaystyle \int_{0}^{1}x^3,dx$

$\displaystyle \int_{0}^{1}(4x)^3,dx$

$\displaystyle \int_{0}^{4}(4x)^3,dx$

$\displaystyle \int_{0}^{4}x^3,dx$

Explanation

This question tests your ability to translate a Riemann sum into its corresponding definite integral notation. The Riemann sum uses $Δx = 4/n$, indicating an interval width of 4. The argument $4i/n$ increases from approximately 0 to 4 as i goes from 1 to n, mapping to the integral limits from 0 to 4. The integrand $x^3$ corresponds directly to the cubed expression in the sum. A tempting distractor is choice B, which fails because using $(4x)^3$ over 0 to 1 scales by $4^3 = 64$ but integrates over width 1, resulting in 64 times the integral $\int_0^1 x^3 , dx$, which is too large. A transferable translation strategy is to identify $Δx$ as $(b - a)/n$, determine a and b from the range of the argument as the index varies from 1 to n, and set the integrand to match the function of that argument.

10

In modeling work, $\sum_{i=1}^{50} h(5+0.2i)(0.2)$ is used. Which integral matches this sum?

$\displaystyle \int_{5.2}^{15.2} h(x),dx$

$\displaystyle \int_{0}^{50} h(5+0.2x),dx$

$\displaystyle \int_{5}^{15} h(x),dx$

$\displaystyle \int_{5}^{15} h(5+0.2x),dx$

$\displaystyle \int_{5}^{15} h(x),(0.2),dx$

Explanation

This question assesses the skill of translating a Riemann sum into its corresponding definite integral. The Riemann sum is \(\sum_{i=1}^{50} h(5+0.2i)(0.2)\), where \(\Delta x = 0.2\) is the width of each subinterval. The evaluation points are 5 + 0.2i for i from 1 to 50, ranging from 5.2 to 15, with 50 terms. This corresponds to a right Riemann sum for \(\int_{5}^{15} h(x),dx\), with total length 50 × 0.2 = 10, from 5 to 15. A tempting distractor like choice D, \(\int_{5.2}^{15.2} h(x),dx\), fails by shifting both limits to the evaluation points without preserving the original interval length. A general strategy for translating Riemann sums is to identify \(\Delta x\), count the terms to find the total length, and determine the limits by aligning the starting point with the overall interval.

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