Concepts and Vocabulary
If an interval \([a,b]\) is partitioned into \(n\) subintervals \([x_{0},\,x_{1}],\) \([x_{1},\,x_{2}],\) \([x_{2},\,x_{3}],\) \(\ldots, \) \([x_{n-1},\,x_{n}]\), where \(a=x_{0}\lt x_{1}\lt x_{2}\lt\cdots \lt x_{n-1}\lt x_{n}=b,\) then the set of subintervals of the interval \([a,b]\) is called a(n) ________ of \([a,b]\).
Multiple Choice In a regular partition of \([0,40] \) into \(20\) subintervals, \( \Delta x= \) [(a) 20 (b) 40 (c) 2 (d) 4].
True or False A function \(f\) defined on the closed interval \([a,b] \) has an infinite number of Riemann sums.
In the notation for a definite integral \(\int_{a}^{b}f(x)\, dx\), \(a\) is called the _______ _______; \(b\) is called the _______ _______; \(\int \) is called the _________ _________; and \(f(x) \) is called the_________.
If \(f(a) \) is defined, \(\int_{a}^{a}f(x)\, dx=\) _________.
True or False If a function \(f\) is integrable over a closed interval \([a,b] \), then \(\int_{a}^{b}f(x) dx=\int_{b}^{a}f(x) dx\).
True or False If a function \(f\) is continuous on a closed interval \([a,b] \), then the definite integral \(\int_{a}^{b}f(x)\, dx\) exists.
Multiple Choice Since \(\int_{0}^{2}(3x-8) dx=-10\), then \(\int_{2}^{0}(3x-8)\, dx=\) [(a) \(-\)2 (b) 10 (c) 5 (d) 0].
Skill Building
In Problems 9–12, find the Riemann sum for each function \(f\) for the partition and the numbers \(u_{i}\) listed.
\({f(x)=x}\), \(0\leq x \leq 2\). Partition the interval \([0,2]\) as follows: \[ \begin{eqnarray*} &&x_{0}=0, x_{1}=\dfrac{1}{4}, x_{2}=\dfrac{1}{2}, x_{3}=\dfrac{3}{4}, x_{4}=1, x_{5}=2;\\ &&\left[0,\dfrac{1}{4}\right], \left[\dfrac{1}{4},\dfrac{1}{2}\right], \left[\dfrac{1}{2},\dfrac{3}{4}\right], \left[\dfrac{3}{4},1\right], [1,2] \end{eqnarray*} \]
and choose \[ u_{1}=\dfrac{1}{8}, u_{2}=\dfrac{3}{8}, u_{3}=\dfrac{5}{8}, u_{4}=\dfrac{7}{8}, u_{5}=\dfrac{9}{8}. \]
\({f(x)=x}\), \(0\leq x \leq 2\). Partition the interval \([0,2]\) as follows: \(\left[0,\dfrac{1}{2}\right]\), \(\left[\dfrac{1}{2},1\right]\), \(\left[1,\dfrac{3}{2}\right]\), \(\left[\dfrac{3}{2},2\right]\), and choose \(u_{1}=\dfrac{1}{2}\), \(u_{2}=1\), \(u_{3}=\dfrac{3}{2}\), \(u_{4}=2\).
\(f(x)=x^{2}\), \(-2\leq x \leq 1\). Partition the interval \([-2,1]\) as follows: \([-2,-1]\), \([-1,0]\), \([0,1]\) and choose \(u_{1}={-}{\dfrac{3}{2}}\), \(u_{2}={-}{\dfrac{1}{2}}\), \(u_{3}={\dfrac{1}{2}}\).
360
\({f(x)=x^{2}}\), \(1\leq x\leq 2\). Partition the interval \([1,2]\) as follows: \(\left[1,\dfrac{5}{4}\right]\), \(\left[\dfrac{5}{4},\dfrac{3}{2}\right]\), \(\left[\dfrac{3}{2},\dfrac{7}{4}\right]\), \(\left[\dfrac{7}{4},2\right]\) and choose \(u_{1}=\dfrac{5}{4}\), \(u_{2}=\dfrac{3}{2}\), \(u_{3}=\dfrac{7}{4}\), \(u_{4}=2\).
In Problems 13 and 14, the graph of a function \(f\) defined on an interval \([a,b] \) is given.
In Problems 15–22, write the limit of the Riemann sums as a definite integral. Here \(u_i\) is in the integral \([x_{i-1}, x_i]\), \(i = 1, 2, \ldots n\).
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}\left(e^{u_{i}}+2\right) \Delta x_{i}\) on \([0,2] \)
\(\lim\limits_{{\max \Delta x}_{i}\rightarrow 0}\sum\limits_{i=1}^{n}\ln u_{i}\Delta x_{i}\) on \([1,8]\)
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}\cos u_{i}\Delta x_{i}\) on \([0,2\pi] \)
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}\left( \cos u_{i}+\sin u_{i}\right) \Delta x_{i}\) on \([0,\pi] \)
\( \lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}{\dfrac{{2}}{{u^{2}_i}}}\Delta x_{i}\) on \([1,4] \)
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}u^{1/3}_i\Delta x_{i}\) on \([0,8]\)
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0}\sum\limits_{i=1}^{n}u_{i}\ln u_{i}\) \(\Delta x_{i}\) on \([1, e]\)
\(\lim\limits_{{\max \Delta x}_{i} \rightarrow 0} \sum\limits_{i=1}^{n}\ln (u_{i}+1)\Delta x_{i}\) on \([0, e] \)
In Problems 23–28, find each definite integral.
\(\int_{-3}^{4}e\,dx \)
\(\int_{0}^{3}(-\pi) \,dx\)
\(\int_{3}^{0}(-\pi)\, dt\)
\(\int_{7}^{2}2\, ds\)
\(\int_{4}^{4}2\, \theta\ d\theta \)
\(\int_{-1}^{-1}8\, dr\)
In Problems 29–32, the graph of a function is shown. Express the shaded area as a definite integral.
In Problems 33–38, determine which of the following definite integrals can be interpreted as area. For those that can, describe the area; for those that cannot, explain why.
\(\int_{0}^{\pi }\sin x\,dx\)
\(\int_{-\pi /4}^{\pi /4}\tan x\,dx\)
\(\int_{1}^{4}(x-2) ^{1/3}dx\)
\(\int_{1}^{4}(x+2) ^{1/3}dx\)
\(\int_{1}^{4}(\vert x \vert \,-2 ) \,dx\)
\(\int_{-2}^{4}\vert x \vert \,\,dx\)
In Problems 39–44:
\(f(x) =x^{2}-1\) on \([0,2]\)
\(f(x) =x^{3}-2\) on \([0,5]\)
\(f(x) =\sqrt{x+1}\) on \([0,3] \)
\(f(x) =\sin x\) on \([0, \pi] \)
\(f(x) =e^{x}\) on \([0, 2] \)
\(f(x) =e^{-x}\) on \([0,1] \)
361
In Problems 45 and 46, find each definite integral using Riemann sums.
\({\int_{0}^{1}(x - 4)dx} \)
\(\int_{0}^{3}{(3x - 1)dx}\)
In Problems 47–50, for each function defined on the interval \([a,b]\):
\(n\) | \(10\) | \(50\) | \(100\) |
Using left endpoints | |||
Using right endpoints | |||
Using the midpoint |
\(f(x) =2+\sqrt{x}\) on \([1,5] \)
\(f(x) =e^{x}+e^{-x}\) on \([-1,3] \)
\(f(x) =\dfrac{3}{1+x^{2}}\) on \([-1,1] \)
\(f(x) = \dfrac{1}{\sqrt{x^2+4}} \) on \([0,2] \)
Applications and Extensions
Find an approximate value of \({\int_{1}^{2}}\dfrac{1}{x}\,{dx}\) by finding Riemann sums corresponding to a partition of \([1,2]\) into four subintervals, each of the same length, and evaluating the integrand at the midpoint of each subinterval. Compare your answer with the true value, \(0.6931\ldots .\)
Units of an Integral In the definite integral \(\int_{0}^{5}F(x)\, dx\), \(F\) represents a force measured in newtons and \(x,\) \(0\leq x\leq 5,\) is measured in meters. What are the units of \(\int_{0}^{5}F(x)\, dx?\)
Units of an Integral In the definite integral \(\int_{0}^{50}C(x)\, dx\), \(C\) represents the concentration of a drug in grams per liter and \(x,\) \(0\leq x\leq 50,\) is measured in liters of alcohol. What are the units of \(\int_{0}^{50}C(x)\, dx?\)
Units of an Integral In the definite integral \(\int_{a}^{b}v(t)\, dt,\) \(v\) represents velocity measured in meters per second and time \(t\) is measured in seconds. What are the units of \(\int_{a}^{b}v(t)\, dt?\)
Units of an Integral In the definite integral \(\int_{a}^{b}S(t)\, dt\), \(S\) represents the rate of sales of a corporation measured in millions of dollars per year and time \(t\) is measured in years. What are the units of \(\int_{a}^{b}S(t)\, dt?\)
Area
Area
The interval \([1,5]\) is partitioned into eight subintervals each of the same length.
Challenge Problems
The floor function \(f(x) = \) \(\lfloor x\rfloor \) is not continuous on \([0,4].\) Show that \({\int_{0}^{4}{f(x)\,dx}}\) exists.
Consider the Dirichlet function \(f,\) where \[ f(x)=\left\{ \begin{array}{@{}l@{ }l@{ }l} {1} & \hbox{if} & {x}~\hbox{is rational} \\ {0} & \hbox{if} & {x}~\hbox{is irrational} \end{array} \right. \] Show that \({\int_{0}^{1}{f(x)\,dx}}\) does not exist. (Hint: Evaluate the Riemann sums in two different ways: first by using rational numbers for \({ u}_{i}\) and then by using irrational number seak for \({ u}_{i}\).)
It can be shown (with a certain amount of work) that if \(f(x)\) is integrable on the interval \([a,b],\) then so is \(\vert f(x) \vert \). Is the converse true?
If only regular partitions are allowed, then we could not always partition an interval \([a,b]\) in a way that automatically partitions subintervals \([a,c] \) and \([c,b]\) for \(a\lt c\lt\,b\). Why not?
If \(f\) is a function that is continuous on a closed interval \([a,b] \), except at \(x_{1},\) \(x_{2},\) \( \ldots \), \(x_{n},\) \(n\geq 1\) an integer, where it has a jump discontinuity, show that \(f\) is integrable on \([a,b] .\)
If \(f\) is a function that is continuous on a closed interval \([a,b] \), except at \(x_{1},\) \(x_{2},\) \( \ldots \), \(x_{n},\) \(n\geq 1\) an integer, where it has a removable discontinuity, show that \(f\) is integrableon \([a,b] .\)