Integration

http://numericalmethods.eng.usf.edu 1
Integration
Topic: Trapezoidal Rule
Major: General Engineering
Author: Autar Kaw, Charlie Barker

http://
numericalmethods.eng.usf.edu2
What is Integration
Integration:
The process of measuring
the area under a function
plotted on a graph.
Where:
f(x) is the integrand
a= lower limit of integration
b= upper limit of integration

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Basis of Trapezoidal Rule
Trapezoidal Rule is based on the Newton-Cotes
Formula that states if one can approximate the
integrand as an nth order polynomial…
where
and

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Basis of Trapezoidal Rule
Then the integral of that function is approximated
by the integral of that nth order polynomial.
Trapezoidal Rule assumes n=1, that is, the area
under the linear polynomial,

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Derivation of the Trapezoidal Rule

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Method Derived From Geometry
The area under the
curve is a trapezoid.
The integral

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Example 1
The vertical distance covered by a rocket from t=8 to
t=30 seconds is given by:
a) Use single segment Trapezoidal rule to find the distance covered.
b) Find the true error, for part (a).
c) Find the absolute relative true error, for part (a).

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Solution
a)

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Solution (cont)
a)
b) The exact value of the above integral is

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Solution (cont)
b)
c) The absolute relative true error, , would be

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Multiple Segment Trapezoidal Rule
In Example 1, the true error using single segment trapezoidal rule was
large. We can divide the interval [8,30] into [8,19] and [19,30] intervals
and apply Trapezoidal rule over each segment.

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With
Hence:

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The true error is:
The true error now is reduced from -807 m to -205 m.
Extending this procedure to divide the interval into equal
segments to apply the Trapezoidal rule; the sum of the
results obtained for each segment is the approximate
value of the integral.

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Figure 4: Multiple (n=4) Segment Trapezoidal Rule
Divide into equal segments
as shown in Figure 4. Then
the width of each segment is:
The integral I is:

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The integral I can be broken into h integrals as:
Applying Trapezoidal rule on each segment gives:

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Example 2
The vertical distance covered by a rocket from to seconds is
given by:
a) Use two-segment Trapezoidal rule to find the distance covered.

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Solution
a) The solution using 2-segment Trapezoidal rule is

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Solution (cont)
Then:

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Solution (cont)
so the true error is

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Solution (cont)
c) The absolute relative true error, , would be

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Solution (cont)
Table 1 gives the values
obtained using multiple
segment Trapezoidal rule
for:
n Value Et
1 11868 -807 7.296 ---
2 11266 -205 1.853 5.343
3 11153 -91.4 0.8265 1.019
4 11113 -51.5 0.4655 0.3594
5 11094 -33.0 0.2981 0.1669
6 11084 -22.9 0.2070 0.09082
7 11078 -16.8 0.1521 0.05482
8 11074 -12.9 0.1165 0.03560
Table 1: Multiple Segment Trapezoidal Rule Values

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Example 3
Use Multiple Segment Trapezoidal Rule to find
the area under the curve
from to
.
Using two segments, we get and

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Solution
Then:

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Solution (cont)
So what is the true value of this integral?
Making the absolute relative true error:

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Solution (cont)
n Approximate
Value
1 0.681 245.91 99.724%
2 50.535 196.05 79.505%
4 170.61 75.978 30.812%
8 227.04 19.546 7.927%
16 241.70 4.887 1.982%
32 245.37 1.222 0.495%
64 246.28 0.305 0.124%
Table 2: Values obtained using Multiple Segment
Trapezoidal Rule for:

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Error in Multiple Segment
Trapezoidal Rule
The true error for a single segment Trapezoidal rule is given by:
where is some point in
What is the error, then in the multiple segment Trapezoidal rule? It will
be simply the sum of the errors from each segment, where the error in
each segment is that of the single segment Trapezoidal rule.
The error in each segment is

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Trapezoidal Rule
Similarly:
It then follows that:

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Trapezoidal Rule
Hence the total error in multiple segment Trapezoidal rule is
The term is an approximate average value of the
.
Hence:

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Trapezoidal Rule
Below is the table for the integral
as a function of the number of segments. You can visualize that as the number
of segments are doubled, the true error gets approximately quartered.
n Value
2 11266 -205 1.854 5.343
4 11113 -51.5 0.4655 0.3594
8 11074 -12.9 0.1165 0.03560
16 11065 -3.22 0.02913 0.00401

Integration
Topic: Simpson’s 1/3rd Rule

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Basis of Simpson’s 1/3rd Rule
Trapezoidal rule was based on approximating the integrand by a first
order polynomial, and then integrating the polynomial in the interval of
integration. Simpson’s 1/3rd rule is an extension of Trapezoidal rule
where the integrand is approximated by a second order polynomial.
Hence
Where is a second order polynomial.

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Choose
and
as the three points of the function to evaluate a0, a1 and a2.

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Solving the previous equations for a0, a1 and a2 give

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Then

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Substituting values of a0, a1, a 2 give
Since for Simpson’s 1/3rd Rule, the interval [a, b] is broken
into 2 segments, the segment width

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Hence
Because the above form has 1/3 in its formula, it is called Simpson’s 1/3rd Rule.

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Example 1
a) Use Simpson’s 1/3rd Rule to find the approximate value of x
The distance covered by a rocket from t=8 to t=30 is given by
b) Find the true error,
c) Find the absolute relative true error,

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Solution
a)

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Solution (cont)
True Error

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Solution (cont)
a)c) Absolute relative true error,

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Multiple Segment Simpson’s
1/3rd Rule

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Multiple Segment Simpson’s 1/3rd
Rule
Just like in multiple segment Trapezoidal Rule, one can subdivide the interval
[a, b] into n segments and apply Simpson’s 1/3rd Rule repeatedly over
every two segments. Note that n needs to be even. Divide interval
[a, b] into equal segments, hence the segment width
where

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Rule
.
.
Apply Simpson’s 1/3rd Rule over each interval,
f(x)
. . .
x0 x2 xn-2 xn
x

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Rule
Since

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Rule
Then

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Rule

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Example 2
Use 4-segment Simpson’s 1/3rd Rule to approximate the distance
covered by a rocket from t= 8 to t=30 as given by
a) Use four segment Simpson’s 1/3rd Rule to find the approximate
value of x.

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Solution
Using n segment Simpson’s 1/3rd Rule,
So
a)

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Solution (cont.)

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Solution (cont.)
cont.

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Solution (cont.)
In this case, the true error isb)
The absolute relative true errorc)

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Solution (cont.)
Table 1: Values of Simpson’s 1/3rd Rule for Example 2 with multiple segments
n Approximate Value Et |Єt |
2
4
6
8
10
11065.72
11061.64
11061.40
11061.35
11061.34
4.38
0.30
0.06
0.01
0.00
0.0396%
0.0027%
0.0005%
0.0001%
0.0000%

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Error in the Multiple Segment
Simpson’s 1/3rd Rule
The true error in a single application of Simpson’s 1/3rd Rule is given as
In Multiple Segment Simpson’s 1/3rd Rule, the error is the sum of the errors
in each application of Simpson’s 1/3rd Rule. The error in n segment Simpson’s
1/3rd Rule is given by

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.
.
.

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Hence, the total error in Multiple Segment Simpson’s 1/3rd Rule is

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The term is an approximate average value of
Hence
where

Integration
Topic: Romberg Rule

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What is The Romberg Rule?
Romberg Integration is an extrapolation formula of
the Trapezoidal Rule for integration. It provides a
better approximation of the integral by reducing the
True Error.

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Trapezoidal Rule
The true error in a multiple segment Trapezoidal
Rule with n segments for an integral
Is given by
where for each i, is a point somewhere in the
domain , .

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Trapezoidal Rule
The term can be viewed as an
approximate average value of in .
This leads us to say that the true error, Et
previously defined can be approximated as

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Trapezoidal Rule
Table 1 shows the results
obtained for the integral
using multiple segment
Trapezoidal rule for
n Value Et
1 11868 807 7.296 ---
2 11266 205 1.854 5.343
3 11153 91.4 0.8265 1.019
4 11113 51.5 0.4655 0.3594
5 11094 33.0 0.2981 0.1669
6 11084 22.9 0.2070 0.09082
7 11078 16.8 0.1521 0.05482
8 11074 12.9 0.1165 0.03560
Table 1: Multiple Segment Trapezoidal Rule Values

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Trapezoidal Rule
The true error gets approximately quartered as
the number of segments is doubled. This
information is used to get a better approximation
of the integral, and is the basis of Richardson’s
extrapolation.

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Richardson’s Extrapolation for
Trapezoidal Rule
The true error, in the n-segment Trapezoidal rule
is estimated as
where C is an approximate constant of
proportionality. Since
Where TV = true value and = approx. value

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Richardson’s Extrapolation for
Trapezoidal Rule
From the previous development, it can be shown
that
when the segment size is doubled and that
which is Richardson’s Extrapolation.

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Example 1
The vertical distance covered by a rocket from 8 to 30
seconds is given by
a) Use Richardson’s rule to find the distance covered.
Use the 2-segment and 4-segment Trapezoidal
rule results given in Table 1.
b) Find the true error, Et for part (a).

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Solution
a)
Using Richardson’s extrapolation formula
for Trapezoidal rule
and choosing n=2,

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Solution (cont.)
Hence

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Solution (cont.)
.
c) The absolute relative true error would then be
Table 2 shows the Richardson’s extrapolation
results using 1, 2, 4, 8 segments. Results are
compared with those of Trapezoidal rule.

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.
Solution (cont.)
Table 2: The values obtained using Richardson’s
extrapolation formula for Trapezoidal rule for
n Trapezoidal
Rule
for Trapezoidal
Rule
Richardson’s
Extrapolation
for Richardson’s
Extrapolation
1
2
4
8
11868
11266
11113
11074
7.296
1.854
0.4655
0.1165
--
11065
11062
11061
--
0.03616
0.009041
0.0000
Table 2: Richardson’s Extrapolation Values

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Romberg Integration
Romberg integration is same as Richardson’s
extrapolation formula as given previously. However,
Romberg used a recursive algorithm for the
extrapolation. Recall
This can alternately be written as

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Note that the variable TV is replaced by as the
value obtained using Richardson’s extrapolation formula.
Note also that the sign is replaced by = sign.
Romberg Integration
Hence the estimate of the true value now is
Where Ch4 is an approximation of the true error.

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Romberg Integration
Determine another integral value with further halving
the step size (doubling the number of segments),
It follows from the two previous expressions
that the true value TV can be written as

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Romberg Integration
The index k represents the order of extrapolation.
k=1 represents the values obtained from the regular
Trapezoidal rule, k=2 represents values obtained using the
true estimate as O(h2). The index j represents the more and
less accurate estimate of the integral.
A general expression for Romberg integration can be
written as

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Example 2
The vertical distance covered by a rocket from
to seconds is given by
Use Romberg’s rule to find the distance covered. Use
the 1, 2, 4, and 8-segment Trapezoidal rule results as
given in the Table 1.

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Solution
From Table 1, the needed values from original
Trapezoidal rule are
where the above four values correspond to using 1, 2,
4 and 8 segment Trapezoidal rule, respectively.

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Solution (cont.)
To get the first order extrapolation values,
Similarly,

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Solution (cont.)
For the second order extrapolation values,
Similarly,

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Solution (cont.)
For the third order extrapolation values,
Table 3 shows these increased correct values in a tree
graph.

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Solution (cont.)
11868
1126
11113
11074
11065
11062
11061
11062
11061
11061
1-segment
2-segment
4-segment
8-segment
First Order Second Order Third Order
Table 3: Improved estimates of the integral value using Romberg Integration

Integration
Topic: Gauss Quadrature Rule of
Integration

Two-Point Gaussian
Quadrature Rule

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Basis of the Gaussian
Quadrature Rule
Previously, the Trapezoidal Rule was developed by the method
of undetermined coefficients. The result of that development is
summarized below.

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Quadrature Rule
The two-point Gauss Quadrature Rule is an extension of the
Trapezoidal Rule approximation where the arguments of the
function are not predetermined as a and b but as unknowns
x1 and x2. In the two-point Gauss Quadrature Rule, the
integral is approximated as

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Quadrature Rule
The four unknowns x1, x2, c1 and c2 are found by assuming that
the formula gives exact results for integrating a general third
order polynomial,
Hence

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Quadrature Rule
It follows that
Equating Equations the two previous two expressions yield

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Quadrature Rule
Since the constants a0, a1, a2, a3 are arbitrary

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Basis of Gauss Quadrature
The previous four simultaneous nonlinear Equations have
only one acceptable solution,

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Basis of Gauss Quadrature
Hence Two-Point Gaussian Quadrature Rule

Higher Point Gaussian
Quadrature Formulas

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Higher Point Gaussian Quadrature
Formulas
is called the three-point Gauss Quadrature Rule.
The coefficients c1, c2, and c3, and the functional arguments x1, x2, and x3
are calculated by assuming the formula gives exact expressions for
General n-point rules would approximate the integral
integrating a fifth order polynomial

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Arguments and Weighing Factors
for n-point Gauss Quadrature
Formulas
In handbooks, coefficients and
Gauss Quadrature Rule are
as shown in Table 1.
Points Weighting
Factors
Function
Arguments
2 c1 = 1.000000000
c2 = 1.000000000
x1 = -0.577350269
x2 = 0.577350269
3 c1 = 0.555555556
c2 = 0.888888889
c3 = 0.555555556
x1 = -0.774596669
x2 = 0.000000000
x3 = 0.774596669
4 c1 = 0.347854845
c2 = 0.652145155
c3 = 0.652145155
c4 = 0.347854845
x1 = -0.861136312
x2 = -0.339981044
x3 = 0.339981044
x4 = 0.861136312
arguments given for n-point
given for integrals
Table 1: Weighting factors c and function
arguments x used in Gauss Quadrature
Formulas.

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Formulas
Points Weighting
Factors
Function
Arguments
5 c1 = 0.236926885
c2 = 0.478628670
c3 = 0.568888889
c4 = 0.478628670
c5 = 0.236926885
x1 = -0.906179846
x2 = -0.538469310
x3 = 0.000000000
x4 = 0.538469310
x5 = 0.906179846
6 c1 = 0.171324492
c2 = 0.360761573
c3 = 0.467913935
c4 = 0.467913935
c5 = 0.360761573
c6 = 0.171324492
x1 = -0.932469514
x2 = -0.661209386
x3 = -0.2386191860
x4 = 0.2386191860
x5 = 0.661209386
x6 = 0.932469514
Table 1 (cont.) : Weighting factors c and function arguments x used in
Gauss Quadrature Formulas.

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Formulas
So if the table is given for integrals, how does one solve
? The answer lies in that any integral with limits of
can be converted into an integral with limits Let
If then
If then
Such that:

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Formulas
Then Hence
Substituting our values of x, and dx into the integral gives us

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Example 1
For an integral derive the one-point Gaussian Quadrature
Rule.
Solution
The one-point Gaussian Quadrature Rule is

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Solution
Assuming the formula gives exact values for integrals
and
Since the other equation becomes

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Solution (cont.)
Therefore, one-point Gauss Quadrature Rule can be expressed as

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Example 2
Use two-point Gauss Quadrature Rule to approximate the distance
covered by a rocket from t=8 to t=30 as given by
Find the true error, for part (a).
Also, find the absolute relative true error, for part (a).
a)
b)
c)

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Solution
First, change the limits of integration from [8,30] to [-1,1]
by previous relations as follows

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Solution (cont)
.
Next, get weighting factors and function argument values from Table 1
for the two point rule,
.

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Solution (cont.)
Now we can use the Gauss Quadrature formula

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Solution (cont)
since

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Solution (cont)
The absolute relative true error, , is (Exact value = 11061.34m)c)
The true error, , isb)

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