. Section 5.3 Evaluating Definite Integrals V63.0121.006/016, Calculus I New York University April 20, 2010 Announcements I April 16: Quiz 4 on §§4.1–4.4 I April 29: Movie Day!! I April 30: Quiz 5 on §§5.1–5.4 I Monday, May 10, 12:00noon (not 10:00am as previously announced) Final Exam . . Image credit: docman . . . . . .
Computing integrals with Riemann sums is like computing derivatives with limits. The calculus of integrals turns out to come from antidifferentiation. This startling fact is the Second Fundamental Theorem of Calculus!
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
.
.
Section 5.3Evaluating Definite Integrals
V63.0121.006/016, Calculus I
New York University
April 20, 2010
AnnouncementsI April 16: Quiz 4 on §§4.1–4.4I April 29: Movie Day!!I April 30: Quiz 5 on §§5.1–5.4I Monday, May 10, 12:00noon (not 10:00am as previouslyannounced) Final Exam
I April 29: Movie Day!!I April 30: Quiz 5 on§§5.1–5.4
I Monday, May 10,12:00noon (not 10:00amas previously announced)Final Exam
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 2 / 48
. . . . . .
Homework: The Good
Most got problems 1 and 3 right.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 3 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Homework: The Bad (steel pipe)
ProblemA steel pipe is being carried down a hallway 9 ft wide. At the end of thehall there is a right-aangled turn into a narrower hallway 6 ft wide.What is the length of the longest pipe that can be carried horizontallyaround the corner?
. .9
.6
.
..θ
..θ
.9
.6
.9 cscθ
.6 secθ
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 4 / 48
. . . . . .
Solution
SolutionThe longest pipe that barely fits is the smallest pipe that almost doesn’tfit. We want to find the minimum value of
f(θ) = a sec θ + b csc θ
on the interval 0 < θ < π/2. (a = 9 and b = 6 in our problem.)
f′(θ) = a sec θ tan θ − b csc θ cot θ
= asin θcos2 θ
− bcos θsin2 θ
=a sin3 θ − b cos3 θ
sin2 θ cos2 θ
So the critical point is when
a sin3 θ = b cos3 θ =⇒ tan3 θ =ba
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 5 / 48
. . . . . .
Finding the minimum
If f′(θ) = a sec θ tan θ − b csc θ cot θ, then
f′′(θ) = a sec θ tan2 θ + a sec3 θ + b csc θ cot2 θ + b csc3 θ
which is positive on 0 < θ < π/2.So the minimum value is
f(θmin) = a sec θmin + b csc θmin
where tan3 θmin =ba
=⇒ tan θmin =
(ba
)1/3.
Using1+ tan2 θ = sec2 θ 1+ cot2 θ = csc2 θ
We get the minimum value is
min = a
√1+
(ba
)2/3+ b
√1+
(ab
)2/3V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 6 / 48
. . . . . .
Simplifying
min = a
√1+
(ba
)2/3+ b
√1+
(ab
)2/3= b
√b2/3
b2/3+
a2/3
b2/3+ a
√a2/3
a2/3+
b2/3
a2/3
=b
b1/3
√b2/3 + a2/3 +
aa1/3
√a2/3 + b2/3
= b2/3√
b2/3 + a2/3 + a2/3√
a2/3 + b2/3
= (b2/3 + a2/3)√
b2/3 + a2/3
= (a2/3 + b2/3)3/2
If a = 9 and b = 6, then min ≈ 21.070.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 7 / 48
. . . . . .
Homework: The Bad (Diving Board)
ProblemIf a diver of mass m stands at the end of a diving board with length Land linear density ρ, then the board takes on the shape of a curvey = f(x), where
EIy′′ = mg(L− x) + 12ρg(L− x)2
E and I are positive constants that depend of the material of the boardand g < 0 is the acceleration due to gravity.(a) Find an expression for the shape of the curve.(b) Use f(L) to estimate the distance below the horizontal at the end of
the board.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 8 / 48
. . . . . .
SolutionWe have
EIy′′(x) = mg(L− x) + 12ρg(L− x)2
Antidifferentiating once gives
EIy′(x) = −12mg(L− x)2 − 1
6ρg(L− x)3 + C
Once more:
EIy(x) = 16mg(L− x)3 + 1
24ρg(L− x)4 + Cx+ D
where C and D are constants.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 9 / 48
. . . . . .
Don't stop there!
Plugging y(0) = 0 into
EIy′(x) = 16mg(L− x)3 + 1
24ρg(L− x)4 + Cx+ D
gives
0 = 16mgL3 + 1
24ρgL4 + D =⇒ D = −1
6mgL3 − 1
24ρgL4
Plugging y′(0) = 0 into
EIy′(x) = −12mg(L− x)2 − 1
6ρg(L− x)3 + C
gives
0 = −12mgL2 − 1
6ρgL3 + C =⇒ C =
12mgL2 +
16ρgL3
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 10 / 48
. . . . . .
Solution completed
So
EIy(x) = 16mg(L− x)3 + 1
24ρg(L− x)4
+
(12mgL2 +
16L3
)x− 1
6mgL3 − 1
24ρgL4
which means
EIy(L) =(12mgL2 +
16L3
)L− 1
6mgL3 − 1
24ρgL4
=12mgL3 +
16L4 − 1
6mgL3 − 1
24ρgL4
=13mgL3 +
18ρgL4
=⇒ y(L) =gL3
EI
(m3
+ρL8
)V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 11 / 48
. . . . . .
Homework: The Ugly
I Some students have gotten their hands on a solution manual andare copying answers word for word.
I This is very easy to catch: the graders are following the samesolution manual.
I This is not very productive: the best you will do is ace 10% of yourcourse grade.
I This is a violation of academic integrity. I do not take it lightly.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 12 / 48
. . . . . .
Objectives
I Use the EvaluationTheorem to evaluatedefinite integrals.
I Write antiderivatives asindefinite integrals.
I Interpret definite integralsas “net change” of afunction over an interval.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 13 / 48
. . . . . .
Outline
Last time: The Definite IntegralThe definite integral as a limitProperties of the integralComparison Properties of the Integral
Evaluating Definite IntegralsThe Theorem of the DayExamples
The Integral as Total Change
Indefinite IntegralsMy first table of integrals
Computing Area with integrals
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 14 / 48
. . . . . .
The definite integral as a limit
DefinitionIf f is a function defined on [a,b], the definite integral of f from a to bis the number ∫ b
af(x)dx = lim
n→∞
n∑i=1
f(ci)∆x
where ∆x =b− an
, and for each i, xi = a+ i∆x, and ci is a point in[xi−1, xi].
TheoremIf f is continuous on [a,b] or if f has only finitely many jumpdiscontinuities, then f is integrable on [a,b]; that is, the definite integral∫ b
af(x) dx exists and is the same for any choice of ci.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 15 / 48
. . . . . .
Notation/Terminology
∫ b
af(x)dx = lim
n→∞
n∑i=1
f(ci)∆x
I∫
— integral sign (swoopy S)
I f(x) — integrandI a and b — limits of integration (a is the lower limit and b theupper limit)
I dx — ??? (a parenthesis? an infinitesimal? a variable?)I The process of computing an integral is called integration
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 16 / 48
. . . . . .
Properties of the integral
Theorem (Additive Properties of the Integral)
Let f and g be integrable functions on [a,b] and c a constant. Then
1.∫ b
ac dx = c(b− a)
2.∫ b
a[f(x) + g(x)] dx =
∫ b
af(x)dx+
∫ b
ag(x)dx.
3.∫ b
acf(x)dx = c
∫ b
af(x)dx.
4.∫ b
a[f(x)− g(x)] dx =
∫ b
af(x)dx−
∫ b
ag(x)dx.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 17 / 48
. . . . . .
More Properties of the Integral
Conventions: ∫ a
bf(x)dx = −
∫ b
af(x)dx∫ a
af(x)dx = 0
This allows us to have
5.∫ c
af(x)dx =
∫ b
af(x)dx+
∫ c
bf(x)dx for all a, b, and c.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 18 / 48
. . . . . .
Definite Integrals We Know So Far
I If the integral computes anarea and we know thearea, we can use that. Forinstance,∫ 1
0
√1− x2 dx =
π
4
I By brute force wecomputed∫ 1
0x2 dx =
13
∫ 1
0x3 dx =
14
..x
.y
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 19 / 48
. . . . . .
Comparison Properties of the Integral
TheoremLet f and g be integrable functions on [a,b].
6. If f(x) ≥ 0 for all x in [a,b], then∫ b
af(x) dx ≥ 0
7. If f(x) ≥ g(x) for all x in [a,b], then∫ b
af(x)dx ≥
∫ b
ag(x)dx
8. If m ≤ f(x) ≤ M for all x in [a,b], then
m(b− a) ≤∫ b
af(x)dx ≤ M(b− a)
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 21 / 48
. . . . . .
Example
Estimate∫ 2
1
1xdx using Property ??.
SolutionSince
1 ≤ x ≤ 2 =⇒ 12≤ 1
x≤ 1
1
we have
12·(2−1) ≤
∫ 2
1
1xdx ≤ 1·(2−1)
or12≤
∫ 2
1
1xdx ≤ 1
(Not a very good estimate)
..x
.y
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 22 / 48
. . . . . .
Example
Estimate∫ 2
1
1xdx using Property ??.
SolutionSince
1 ≤ x ≤ 2 =⇒ 12≤ 1
x≤ 1
1
we have
12·(2−1) ≤
∫ 2
1
1xdx ≤ 1·(2−1)
or12≤
∫ 2
1
1xdx ≤ 1
(Not a very good estimate)
..x
.y
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 22 / 48
. . . . . .
Outline
Last time: The Definite IntegralThe definite integral as a limitProperties of the integralComparison Properties of the Integral
Evaluating Definite IntegralsThe Theorem of the DayExamples
The Integral as Total Change
Indefinite IntegralsMy first table of integrals
Computing Area with integrals
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 23 / 48
. . . . . .
Socratic dialogue
I The definite integral ofvelocity measuresdisplacement (netdistance)
I The derivative ofdisplacement is velocity
I So we can computedisplacement with thedefinite integral or anantiderivative of velocity
I But any function can be avelocity function, so . . .
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 24 / 48
. . . . . .
Theorem of the Day
Theorem (The Second Fundamental Theorem of Calculus)
Suppose f is integrable on [a,b] and f = F′ for another function F, then∫ b
af(x)dx = F(b)− F(a).
NoteIn Section 5.3, this theorem is called “The Evaluation Theorem”.Nobody else in the world calls it that.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 25 / 48
. . . . . .
Theorem of the Day
Theorem (The Second Fundamental Theorem of Calculus)
Suppose f is integrable on [a,b] and f = F′ for another function F, then∫ b
af(x)dx = F(b)− F(a).
NoteIn Section 5.3, this theorem is called “The Evaluation Theorem”.Nobody else in the world calls it that.
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 25 / 48
. . . . . .
Proving the Second FTC
Proof.
Divide up [a,b] into n pieces of equal width ∆x =b− an
as usual. Foreach i, F is continuous on [xi−1, xi] and differentiable on (xi−1, xi). Sothere is a point ci in (xi−1, xi) with
F(xi)− F(xi−1)
xi − xi−1= F′(ci) = f(ci)
Orf(ci)∆x = F(xi)− F(xi−1)
See if you can spot the invocation of the Mean Value Theorem!
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 26 / 48
. . . . . .
Proving the Second FTC
Proof.
Divide up [a,b] into n pieces of equal width ∆x =b− an
as usual. Foreach i, F is continuous on [xi−1, xi] and differentiable on (xi−1, xi). Sothere is a point ci in (xi−1, xi) with
F(xi)− F(xi−1)
xi − xi−1= F′(ci) = f(ci)
Orf(ci)∆x = F(xi)− F(xi−1)
See if you can spot the invocation of the Mean Value Theorem!
V63.0121.006/016, Calculus I (NYU) Section 5.3 Evaluating Definite Integrals April 20, 2010 26 / 48