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In this section, we will learn:
To substitute a new variable in place of an existing expression in a function, making integration easier.
2.
5.5
The Substitution Rule
In this section, we will learn:
To substitute a new variable in place of an existing
expression in a function, making integration easier.
3.
Due to the Fundamental Theorem
of Calculus (FTC), it’s important to be
able to find antiderivatives.
4.
INTRODUCTION Equation 1
However, our antidifferentiation formulas
don’t tell us how to evaluate integrals such
2
2 x 1 x dx
5.
To find this integral, we use the problem-
solving strategy of introducing something
The ‘something extra’ is a new variable.
We change from the variable x to a new variable u.
6.
Suppose we let u be the quantity under
the root sign in Equation 1, u = 1 + x2.
Then, the differential of u is du = 2x dx.
7.
Notice that, if the dx in the notation for
an integral were to be interpreted as
a differential, then the differential 2x dx
would occur in Equation 1.
8.
INTRODUCTION Equation 2
So, formally, without justifying our calculation,
we could write:
2 2
2 x 1 x dx 1 x 2 x dx
udu
3/ 2
u
2
3 C
2 3/ 2
( x 1)
2
3 C
9.
However, now we can check that we have
the correct answer by using the Chain Rule
to differentiate the final function of Equation 2:
d 2 2
3 ( x 1)3 2 C 23 23 ( x 2 1)1 2 2 x
dx
2
2 x x 1
10.
In general, this method works whenever
we have an integral that we can write in
the form
∫ f(g(x))g’(x) dx
11.
INTRODUCTION Equation 3
Observe that, if F’ = f, then
∫ F’(g(x))g’(x) dx = F(g(x)) + C
because, by the Chain Rule,
d
F ( g ( x)) F '( g ( x)) g '( x)
dx
12.
If we make the ‘change of variable’ or
‘substitution’ u = g(x), from Equation 3,
we have: F '( g ( x)) g '( x) dx F ( g ( x)) C
F (u ) C
F '(u ) du
13.
Writing F’ = f, we get:
∫ f(g(x))g’(x) dx = ∫ f(u) du
Thus, we have proved the following rule.
14.
SUBSTITUTION RULE Equation 4
If u = g(x) is a differentiable function whose
range is an interval I and f is continuous
on I, then
∫ f(g(x))g’(x) dx = ∫ f(u) du
15.
SUBSTITUTION RULE
Notice that the Substitution Rule for
integration was proved using the Chain Rule
for differentiation.
Notice also that, if u = g(x), then du = g’(x) dx.
So, a way to remember the Substitution Rule is
to think of dx and du in Equation 4 as differentials.
16.
SUBSTITUTION RULE
Thus, the Substitution Rule says:
It is permissible to operate with
dx and du after integral signs as if
they were differentials.
17.
SUBSTITUTION RULE Example 1
Find ∫ x3 cos(x4 + 2) dx
We make the substitution u = x4 + 2.
This is because its differential is du = 4x3 dx,
which, apart from the constant factor 4,
occurs in the integral.
18.
SUBSTITUTION RULE Example 1
Thus, using x3 dx = du/4 and the Substitution
Rule, we have:
3 4
x cos( x 2) dx cos
4 u 1
du 1
4 cos u du
14 sin u C
4
sin( x 2) C
1
4
Notice that, at the final stage, we had to return to
the original variable x.
19.
SUBSTITUTION RULE
The idea behind the Substitution Rule is to
replace a relatively complicated integral by
a simpler integral.
This is accomplished by changing from the original
variable x to a new variable u that is a function of x.
Thus, in Example 1, we replaced the integral
∫ x3cos(x4 + 2) dx by the simpler integral ¼ ∫ cos u du.
20.
SUBSTITUTION RULE
The main challenge in using the rule is
to think of an appropriate substitution.
You should try to choose u to be some function in
the integrand whose differential also occurs—except
for a constant factor.
This was the case in Example 1.
21.
SUBSTITUTION RULE
If that is not possible, try choosing u to be
some complicated part of the integrand—
perhaps the inner function in a composite
22.
SUBSTITUTION RULE
Finding the right substitution is
a bit of an art.
It’s not unusual to guess wrong.
If your first guess doesn’t work, try another
substitution.
23.
SUBSTITUTION RULE E. g. 2—Solution 1
Evaluate 2 x 1 dx
Let u = 2x + 1.
Then, du = 2 dx.
So, dx = du/2.
24.
SUBSTITUTION RULE E. g. 2—Solution 1
Thus, the rule gives:
du
2 x 1 dx u
2
12
1
2 u du
32
u
1
2 C
3/ 2
1 32
3 u C
32
(2 x 1)
1
3 C
25.
SUBSTITUTION RULE E. g. 2—Solution 2
Another possible substitution is u 2 x 1
dx
Then, du
2 x 1
So, dx 2 x 1
Alternatively, observe that u2 = 2x + 1.
So, 2u du = 2 dx.
26.
SUBSTITUTION RULE E. g. 2—Solution 2
Thus, 2 x 1 dx u u du
2
u du
3
u
C
3
32
3 (2 x 1) C
1
27.
SUBSTITUTION RULE Example 3
x
Find 1 4x dx
2
Let u = 1 – 4x2.
Then, du = -8x dx.
So, x dx = -1/8 du and
x 1 1 2
dx du
1
8
1
8 du
u
1 4 x2 u
18 (2 u ) C 1
4 1 4x2 C
28.
SUBSTITUTION RULE
The answer to the example could be
checked by differentiation.
Instead, let’s check it with a graph.
29.
SUBSTITUTION RULE
Here, we have used a computer to graph
2
both the integrand f ( x ) x / 1 4 x
2
and its indefinite integral g ( x) 4 1 4 x
1
We take the case
C = 0.
30.
SUBSTITUTION RULE
Notice that g(x):
Decreases when f(x) is negative
Increases when f(x) is positive
Has its minimum value when f(x) = 0
31.
SUBSTITUTION RULE
So, it seems reasonable, from the graphical
evidence, that g is an antiderivative of f.
32.
SUBSTITUTION RULE Example 4
Calculate ∫ e5x dx
If we let u = 5x, then du = 5 dx.
So, dx = 1/5 du.
Therefore, 5x u
e dx e du
5
1
u
e C
1
5
5x
e C
1
5
33.
SUBSTITUTION RULE Example 5
2 5
Find 1 x x dx
An appropriate substitution becomes more obvious
if we factor x5 as x4 . x.
Let u = 1 + x2.
Then, du = 2x dx.
So, x dx = du/2.
34.
SUBSTITUTION RULE Example 5
Also, x2 = u – 1; so, x4 = (u – 1)2:
2 5 2 4 du
2
1 x x dx 1 x x x dx u (u 1)
2
12 u (u 2 2u 1) du
5/ 2 3/ 2 1/ 2
1
2 (u 2u u ) du
7/2 5/ 2 3/ 2
( u
1
2
2
7 2 u 2
5 u2
3 ) C
17 (1 x 2 )7 / 2 52 (1 x 2 )5/ 2
13 (1 x 2 )3/ 2 C
35.
SUBSTITUTION RULE Example 6
Calculate ∫ tan x dx
First, we write tangent in terms of sine and cosine:
sin x
tan x dx cos x dx
This suggests that we should substitute u = cos x,
since then du = – sin x dx, and so sin x dx = – du:
sin x du
tan x dx cos x dx u
ln | u | C
ln | cos x | C
36.
SUBSTITUTION RULE Equation 5
Since –ln |cos x| = ln(|cos x|-1)
= ln(1/|cos x|)
= ln|sec x|,
the result of the example can also be written
as ∫ tan x dx = ln |sec x| + C
37.
DEFINITE INTEGRALS
When evaluating a definite integral
by substitution, two methods are
38.
DEFINITE INTEGRALS
One method is to evaluate the indefinite
integral first and then use the FTC.
For instance, using the result of Example 2,
we have: 4 4
2 x 1 dx 2 x 1 dx
0 0
32 4
(2 x 1)
1
3 0
32 32
(9)
1
3 (1)
1
3
13 (27 1) 263
39.
DEFINITE INTEGRALS
Another method, which is usually preferable,
is to change the limits of integration when
the variable is changed.
Thus, we have the substitution rule for
definite integrals.
40.
SUB. RULE FOR DEF. INTEGRALS Equation 6
If g’ is continuous on [a, b] and f
is continuous on the range of u = g(x),
b g (b )
f ( g ( x)) g '( x)dx
a g (a)
f (u )du
41.
SUB. RULE FOR DEF. INTEGRALS Proof
Let F be an antiderivative of f.
Then, by Equation 3, F(g(x)) is an antiderivative
of f(g(x))g’(x).
So, by Part 2 of the FTC (FTC2), we have:
b b
f ( g ( x)) g '( x)dx F ( g ( x))
a a
F ( g (b)) F ( g (a ))
42.
SUB. RULE FOR DEF. INTEGRALS Proof
However, applying the FTC2 a second time,
we also have:
g (b ) g (b )
f (u ) du F (u ) g ( a )
g (a)
F ( g (b)) F ( g (a ))
43.
SUB. RULE FOR DEF. INTEGRALS Example 7
4
Evaluate 2 x 1 dx using Equation 6.
0
Using the substitution
from Solution 1 of
Example 2, we have:
u = 2x + 1 and dx = du/2
44.
SUB. RULE FOR DEF. INTEGRALS Example 7
To find the new limits of integration, we
note that:
When x = 0, u = 2(0) + 1 = 1
When x = 4, u = 2(4) + 1 = 9
45.
SUB. RULE FOR DEF. INTEGRALS Example 7
4 9
Thus, 2 x 1 dx 12 u du
0 1
32 9
12 23 u
1
32 32
(91
3 1 )
26
3
46.
SUB. RULE FOR DEF. INTEGRALS Example 7
Observe that, when using Equation 6,
we do not return to the variable x after
We simply evaluate the expression in u
between the appropriate values of u.
47.
SUB. RULE FOR DEF. INTEGRALS Example 8
2 dx
Evaluate
1 (3 5 x ) 2
Let u = 3 - 5x.
Then, du = – 5 dx, so dx = – du/5.
When x = 1, u = – 2, and when x = 2, u = – 7.
48.
SUB. RULE FOR DEF. INTEGRALS Example 8
2 dx 1 7 du
Thus, 2
2
1 (3 5 x ) 5 2 u
7
1 1
5 u 5
7
1
5u 2
1 1 1 1
5 7 2 14
49.
SUB. RULE FOR DEF. INTEGRALS Example 9
ln xe
Calculate dx
1 x
We let u = ln x because its differential du = dx/x
occurs in the integral.
When x = 1, u = ln 1, and when x = e, u = ln e = 1.
2 1
ln
e x 1 u 1
Thus,
1 x dx 0 u du 2 2
0
50.
SUB. RULE FOR DEF. INTEGRALS Example 9
As the function f(x) = (ln x)/x in the example
is positive for x > 1, the integral represents
the area of the shaded region in this figure.
51.
The next theorem uses the Substitution
Rule for Definite Integrals to simplify
the calculation of integrals of functions that
possess symmetry properties.
52.
INTEGS. OF SYMM. FUNCTIONS Theorem 7
Suppose f is continuous on [–a , a].
a.If f is even, [f(–x) = f(x)], then
a a
f ( x) dx 2 f ( x) dx
a 0
b.If f is odd, [f(-x) = -f(x)], then
a
f ( x) dx 0
a
53.
INTEGS. OF SYMM. FUNCTIONS Proof—Equation 8
We split the integral in two:
a 0 a
f ( x) dx f ( x) dx f ( x) dx
a a 0
a a
f ( x) dx f ( x) dx
0 0
54.
INTEGS. OF SYMM. FUNCTIONS Proof
a 0 a
f ( x) dx f ( x) dx f ( x) dx
a a 0
a a
f ( x) dx f ( x) dx
0 0
In the first integral in the second part,
we make the substitution u = –x .
Then, du = –dx, and when x = –a, u = a.
55.
INTEGS. OF SYMM. FUNCTIONS Proof
a a
f ( x) dx f ( u )( du )
0 0
a
f ( u ) du
0
56.
INTEGS. OF SYMM. FUNCTIONS Proof—Equation 9
So, Equation 8 becomes:
a
f ( x) dx
a
a a
f ( u ) du f ( x) dx
0 0
57.
INTEGS. OF SYMM. FUNCTIONS Proof a
If f is even, then f(–u) = f(u).
a
So, Equation 9 gives: f ( x) dx
a
a a
f (u ) du f ( x) dx
0 0
a
2 f ( x) dx
0
58.
INTEGS. OF SYMM. FUNCTIONS Proof b
If f is odd, then f(–u) = –f(u).
a
So, Equation 9 gives: f ( x) dx
a
a a
f (u ) du f ( x) dx
0 0
0
59.
INTEGS. OF SYMM. FUNCTIONS
Theorem 7 is
illustrated here.
60.
INTEGS. OF SYMM. FUNCTIONS
For the case where
f is positive and
even, part (a) says
that the area under
y = f(x) from –a to a
is twice the area
from 0 to a because
of symmetry.
61.
INTEGS. OF SYMM. FUNCTIONS
b
Recall that an integral f ( x) dx can be
a
expressed as the area above the x–axis
and below y = f(x) minus the area below
the axis and above the curve.
62.
INTEGS. OF SYMM. FUNCTIONS
Therefore, part (b) says the integral
is 0 because the areas cancel.
63.
INTEGS. OF SYMM. FUNCTIONS Example 10
As f(x) = x6 + 1 satisfies f(–x) = f(x), it is even.
2 2
6 6
So, (x
2
1) dx 2 ( x 1) dx
0
7 2
2 x x
1
7 0
2 128
7 2
284
7
64.
INTEGS. OF SYMM. FUNCTIONS Example 11
As f(x) = (tan x)/ (1 + x2 + x4) satisfies
f(–x) = –f(x), it is odd.
1 tan x
So, 2 4
dx 0
11 x x
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