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Trapezoidal Rule of Integration

Civil Engineering Majors

Authors: Autar Kaw, Charlie Barker

http://numericalmethods.eng.usf.eduTransforming Numerical Methods Education for STEM

Undergraduates

Trapezoidal Rule of Integration

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What is IntegrationIntegration:

b

a

dx)x(fI

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

f(x)

a b

b

a

dx)x(f

y

x

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Basis of Trapezoidal Rule

b

a

dx)x(fI

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

where )x(f)x(f n

nn

nnn xaxa...xaa)x(f

1110and

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Basis of Trapezoidal Rule

b

an

b

a

)x(f)x(f

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,

2)b(f)a(f

)ab(b

a

dx)x(f

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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

trapezoidofAreadxxfb

a

)(

)height)(sidesparallelofSum(2

1

)ab()a(f)b(f 2

1

2)b(f)a(f

)ab(

Figure 2: Geometric Representation

f(x)

a b

b

a

dx)x(f1

y

x

f1(x)

Example 1

Since decays rapidly as , we will approximate

a) Use single segment Trapezoidal rule to find the value of erfc(0.6560).

b) Find the true error, for part (a).c) Find the absolute relative true error, for part (a).

6560.0

5

2

6560.0 dzeerfc z

The concentration of benzene at a critical location is given by

758.56560.075.1 73.32 erfceerfcc

where dzexerfcx

z

2

So in the above formula dzeerfc z

6560.0

2

6560.0

tE

a

2ze z

Solution

2

)b(f)a(f)ab(Ia)

2

)( zezf

115 103888.1)5(2 ef

65029.0)6560.0(26560.0 ef

5a

6560.0b

Solution (cont)

a)

b)The exact value of the above integral cannot be found. We assume the value obtained by adaptive numerical integration using Maple as the exact value for calculating the true error and relative true error.

4124.1

2

65029.0103888.156560.0

11

I

31333.0

6560.06560.0

5

2

dzeerfc z

Solution (cont)b) ValueeApproximatValueTrueEt

c) The absolute relative true error, , would be t

)4124.1(31333.0

0991.1

%79.350

10031333.0

4124.131333.0

100Value True

Error True

t

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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.

t.t

ln)t(f 892100140000

1400002000

30

19

19

8

30

8

dt)t(fdt)t(fdt)t(f

2

30191930

2

198819

)(f)(f)(

)(f)(f)(

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Multiple Segment Trapezoidal Rule

With

s/m.)(f 271778

s/m.)(f 7548419

s/m.)(f 6790130

2

67.90175.484)1930(

2

75.48427.177)819()(

30

8

dttf

m11266

Hence:

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Multiple Segment Trapezoidal Rule

1126611061 tE

m205

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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Multiple Segment Trapezoidal Rule

f(x)

a b

y

x

4

aba

4

2ab

a

4

3ab

a

Figure 4: Multiple (n=4) Segment Trapezoidal Rule

Divide into equal segments as shown in Figure 4. Then the width of each segment is:

n

abh

The integral I is:

b

a

dx)x(fI

b

h)n(a

h)n(a

h)n(a

ha

ha

ha

a

dx)x(fdx)x(f...dx)x(fdx)x(f1

1

2

2

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Multiple Segment Trapezoidal Rule

The integral I can be broken into h integrals as:

b

a

dx)x(f

Applying Trapezoidal rule on each segment gives:

b

a

dx)x(f

)b(f)iha(f)a(f

n

ab n

i

1

1

22

Example 2

Since decays rapidly as , we will approximate

a) Use two-segment Trapezoidal rule to find the value of erfc(0.6560).

b) Find the true error, for part (a).c) Find the absolute relative true error, for part (a).

6560.0

5

2

6560.0 dzeerfc z

The concentration of benzene at a critical location is given by 758.56560.075.1 73.32 erfceerfcc

where dzexerfcx

z

2

So in the above formula dzeerfc z

6560.0

2

6560.0

atE

2ze z

Solutiona) The solution using 2-segment Trapezoidal rule is

)()(2)(2

1

1

bfihafafn

abI

n

i

2n

5a

6560.0b

172.22

56560.0

n

abh

Solution (cont)Then:

70695.0

65029.000033627.02103888.14

344.4

6560.0828.2254

344.4

6560.02522

56560.0

11

12

1

fff

fihaffIi

Solution (cont)b) The exact value of the above integral cannot be found. We assume the value obtained by adaptive numerical integration using Maple as the exact value for calculating the true error and relative true error.

so the true error is

39362.0

70695.031333.0

ValueeApproximatValueTrueEt

31333.06560.06560.0

5

2

dzeerfc z

Solution (cont)

c)The absolute relative true error, , would be t

%63.125

10031333.0

70695.031333.0

100Value True

Error True

t

Solution (cont)

Table 1 gives the values obtained using

multiple segment Trapezoidal rule for:

Table 1: Multiple Segment Trapezoidal Rule Values

6560.0

5

2

6560.0 dzeerfc z

%t %an Value Et

1 −1.4124 1.0991 350.79 ---

2 −0.70695 0.39362 125.63 99.793

3 −0.48812 0.17479 55.787 44.829

4 −0.40571 0.092379 29.483 20.314

5 −0.37028 0.056957 18.178 9.5662

6 −0.35212 0.038791 12.380 5.1591

7 −0.34151 0.028182 8.9946 3.1063

8 −0.33475 0.021426 6.8383 2.0183

Example 3

Use Multiple Segment Trapezoidal Rule to find the area under the curve

xe

x)x(f

1

300from to 0x 10x

.

Using two segments, we get 52

010 h

01

03000

0

e

)()(f 03910

1

53005

5.

e

)()(f

1360

1

1030010

10.

e

)()(f

and

Solution

)b(f)iha(f)a(f

n

abI

n

i

1

12

2

)(f)(f)(f

)( i105020

22

010 12

1

)(f)(f)(f 10520

4

10

136003910204

10.).(

53550.

Then:

Solution (cont)

So what is the true value of this integral?

592461

30010

0

.dxe

xx

Making the absolute relative true error:

%.

..t 100

59246

5355059246

%.50679

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:

10

01

300dx

e

xx

tE t

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Error in Multiple Segment Trapezoidal Rule

The true error for a single segment Trapezoidal rule is given by:

ba),("f)ab(

Et

12

3

where is some point in b,a

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

haa),("f

a)ha(E

11

3

1 12

)("fh

1

3

12

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Error in Multiple Segment Trapezoidal Rule

Similarly:

ihah)i(a),("f

)h)i(a()iha(E iii

1

12

1 3

)("fh

i12

3

It then follows that:

bh)n(a),("f

h)n(abE nnn

1

12

1 3

)("fh

n12

3

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Error in Multiple Segment Trapezoidal Rule

Hence the total error in multiple segment Trapezoidal rule is

n

iit EE

1

n

ii )("f

h

1

3

12 n

)("f

n

)ab(

n

ii

1

2

3

12

The term n

)("fn

ii

1is an approximate average value of the bxa),x("f

Hence:

n

)("f

n

)ab(E

n

ii

t

12

3

12

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Error in Multiple Segment Trapezoidal Rule

Below is the table for the integral

30

8

892100140000

1400002000 dtt.

tln

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.

tE %t %an 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

Additional ResourcesFor all resources on this topic such as digital audiovisual lectures, primers, textbook chapters, multiple-choice tests, worksheets in MATLAB, MATHEMATICA, MathCad and MAPLE, blogs, related physical problems, please visit

http://numericalmethods.eng.usf.edu/topics/trapezoidal_rule.html

THE END

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