by Lale Yurttas, Texas A&M University

Chapter 21 1

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Numerical Differentiation and Integration

• Standing in the heart of calculus are the mathematical concepts of differentiation and integration:

b

a

iix

ii

dxxfI

xxfxxf

dxdy

xxfxxf

xy

)(

)()(lim

)()(

0

by Lale Yurttas, Texas A&M University

Chapter 21 2

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Figure PT6.1

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Chapter 21 3

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Figure PT6.2

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Chapter 21 4

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Noncomputer Methods for Differentiation and Integration

• The function to be differentiated or integrated will typically be in one of the following three forms:– A simple continuous function such as polynomial,

an exponential, or a trigonometric function.– A complicated continuous function that is difficult

or impossible to differentiate or integrate directly.– A tabulated function where values of x and f(x) are

given at a number of discrete points, as is often the case with experimental or field data.

by Lale Yurttas, Texas A&M University

Chapter 21 5

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Figure PT6.4

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Chapter 21 6

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Figure PT6.7

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Chapter 21 7

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Newton-Cotes Integration FormulasChapter 21

• The Newton-Cotes formulas are the most common numerical integration schemes.

• They are based on the strategy of replacing a complicated function or tabulated data with an approximating function that is easy to integrate:

nn

nnn

b

an

b

a

xaxaxaaxf

dxxfdxxfI

1

110)(

)()(

by Lale Yurttas, Texas A&M University

Chapter 21 8

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Figure 21.1

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Chapter 21 9

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Figure 21.2

by Lale Yurttas, Texas A&M University

Chapter 21 10

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The Trapezoidal Rule• The Trapezoidal rule is the first of the Newton-Cotes

closed integration formulas, corresponding to the case where the polynomial is first order:

• The area under this first order polynomial is an estimate of the integral of f(x) between the limits of a and b:

b

a

b

a

dxxfdxxfI )()( 1

2)()()( bfafabI

Trapezoidal rule

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Chapter 21 11

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Figure 21.4

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Chapter 21 12

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Error of the Trapezoidal Rule/

• When we employ the integral under a straight line segment to approximate the integral under a curve, error may be substantial:

where lies somewhere in the interval from a to b.

3))((121 abfEt

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Chapter 21 13

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The Multiple Application Trapezoidal Rule/

• One way to improve the accuracy of the trapezoidal rule is to divide the integration interval from a to b into a number of segments and apply the method to each segment.

• The areas of individual segments can then be added to yield the integral for the entire interval.

Substituting the trapezoidal rule for each integral yields:

n

n

x

x

x

x

x

x

n

dxxfdxxfdxxfI

xbxan

abh

1

2

1

1

0

)()()(

0

2)()(

2)()(

2)()( 12110 nn xfxfhxfxfhxfxfhI

by Lale Yurttas, Texas A&M University

Chapter 21 14

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Figure 21.8

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Chapter 21 15

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Simpson’s Rules

• More accurate estimate of an integral is obtained if a high-order polynomial is used to connect the points. The formulas that result from taking the integrals under such polynomials are called Simpson’s rules.

Simpson’s 1/3 Rule/• Results when a second-order interpolating

polynomial is used.

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Figure 21.10

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Chapter 21 17

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2

)()(4)(3

)())((

))(()())((

))(()())((

))((

)()(

210

21202

101

2101

200

2010

21

20

2

2

0

abhxfxfxfhI

dxxfxxxx

xxxxxfxxxx

xxxxxfxxxx

xxxxI

xbxa

dxxfdxxfI

x

x

b

a

b

a

Simpson’s 1/3 Rule