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8/8/2015 http://numericalmethods.eng.usf.edu 1 Gauss Quadrature Rule of Integration Major: All Engineering Majors Authors: Autar Kaw, Charlie Barker http://numericalmethods.eng.usf.edu Transforming Numerical Methods Education for STEM Undergraduates

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Gauss Quadrature Rule of Integration http://numericalmethods.eng.usf.edu http://numericalmethods.eng.usf.edu

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3 What is Integration? Integration The process of measuring the area under a curve. Where: f(x) is the integrand a= lower limit of integration b= upper limit of integration f(x) ab y x

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http://numericalmethods.eng.usf.edu4 Two-Point Gaussian Quadrature Rule

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http://numericalmethods.eng.usf.edu5 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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http://numericalmethods.eng.usf.edu6 Basis of the Gaussian 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 x 1 and x 2. In the two-point Gauss Quadrature Rule, the integral is approximated as

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http://numericalmethods.eng.usf.edu7 Basis of the Gaussian Quadrature Rule The four unknowns x 1, x 2, c 1 and c 2 are found by assuming that the formula gives exact results for integrating a general third order polynomial, Hence

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http://numericalmethods.eng.usf.edu8 Basis of the Gaussian Quadrature Rule It follows that Equating Equations the two previous two expressions yield

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http://numericalmethods.eng.usf.edu9 Basis of the Gaussian Quadrature Rule Since the constants a 0, a 1, a 2, a 3 are arbitrary

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http://numericalmethods.eng.usf.edu10 Basis of Gauss Quadrature The previous four simultaneous nonlinear Equations have only one acceptable solution,

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http://numericalmethods.eng.usf.edu11 Basis of Gauss Quadrature Hence Two-Point Gaussian Quadrature Rule

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http://numericalmethods.eng.usf.edu12 Higher Point Gaussian Quadrature Formulas

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http://numericalmethods.eng.usf.edu13 Higher Point Gaussian Quadrature Formulas is called the three-point Gauss Quadrature Rule. The coefficients c 1, c 2, and c 3, and the functional arguments x 1, x 2, and x 3 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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http://numericalmethods.eng.usf.edu14 Arguments and Weighing Factors for n-point Gauss Quadrature Formulas In handbooks, coefficients and Gauss Quadrature Rule are as shown in Table 1. PointsWeighting Factors Function Arguments 2c 1 = 1.000000000 c 2 = 1.000000000 x 1 = -0.577350269 x 2 = 0.577350269 3c 1 = 0.555555556 c 2 = 0.888888889 c 3 = 0.555555556 x 1 = -0.774596669 x 2 = 0.000000000 x 3 = 0.774596669 4c 1 = 0.347854845 c 2 = 0.652145155 c 3 = 0.652145155 c 4 = 0.347854845 x 1 = -0.861136312 x 2 = -0.339981044 x 3 = 0.339981044 x 4 = 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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http://numericalmethods.eng.usf.edu15 Arguments and Weighing Factors for n-point Gauss Quadrature Formulas PointsWeighting Factors Function Arguments 5c 1 = 0.236926885 c 2 = 0.478628670 c 3 = 0.568888889 c 4 = 0.478628670 c 5 = 0.236926885 x 1 = -0.906179846 x 2 = -0.538469310 x 3 = 0.000000000 x 4 = 0.538469310 x 5 = 0.906179846 6c 1 = 0.171324492 c 2 = 0.360761573 c 3 = 0.467913935 c 4 = 0.467913935 c 5 = 0.360761573 c 6 = 0.171324492 x 1 = -0.932469514 x 2 = -0.661209386 x 3 = -0.2386191860 x 4 = 0.2386191860 x 5 = 0.661209386 x 6 = 0.932469514 Table 1 (cont.) : Weighting factors c and function arguments x used in Gauss Quadrature Formulas.

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http://numericalmethods.eng.usf.edu16 Arguments and Weighing Factors for n-point Gauss Quadrature Formulas So if the table is given forintegrals, how does one solve ?The answer lies in that any integral with limits of can be converted into an integral with limitsLet If then Such that:

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http://numericalmethods.eng.usf.edu17 Arguments and Weighing Factors for n-point Gauss Quadrature Formulas ThenHence Substituting our values of x, and dx into the integral gives us

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http://numericalmethods.eng.usf.edu18 Example 1 For an integralderive the one-point Gaussian Quadrature Rule. Solution The one-point Gaussian Quadrature Rule is

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http://numericalmethods.eng.usf.edu19 Solution The two unknowns x 1, and c 1 are found by assuming that the formula gives exact results for integrating a general first order polynomial,

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http://numericalmethods.eng.usf.edu20 Solution It follows that Equating Equations, the two previous two expressions yield

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http://numericalmethods.eng.usf.edu21 Basis of the Gaussian Quadrature Rule Since the constants a 0, and a 1 are arbitrary giving

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http://numericalmethods.eng.usf.edu22 Solution Hence One-Point Gaussian Quadrature Rule

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http://numericalmethods.eng.usf.edu23 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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http://numericalmethods.eng.usf.edu24 Solution First, change the limits of integration from [8,30] to [-1,1] by previous relations as follows

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http://numericalmethods.eng.usf.edu25 Solution (cont) Next, get weighting factors and function argument values from Table 1 for the two point rule,

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http://numericalmethods.eng.usf.edu26 Solution (cont.) Now we can use the Gauss Quadrature formula

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http://numericalmethods.eng.usf.edu27 Solution (cont) since

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http://numericalmethods.eng.usf.edu28 Solution (cont) The absolute relative true error,, is (Exact value = 11061.34m) c) The true error,, isb)

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Additional Resources For 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/gauss_qua drature.html

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