# hw6_solutions - CS 257 Numerical Methods - Homework 6...

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Unformatted text preview: CS 257 Numerical Methods - Homework 6 October 25, 2006 1. [1pt] Section 4.1 #1 Solution: p ( x ) = 7 ( x- 2)( x- 3)( x- 4) (0- 2)(0- 3)(0- 4) +11 ( x- 0)( x- 3)( x- 4) (2- 0)(2- 3)(2- 4) +28 ( x- 0)( x- 2)( x- 4) (3- 0)(3- 2)(3- 4) +63 ( x- 0)( x- 2)( x- 3) (4- 0)(4- 2)(4- 3) 2. [1pt] Section 4.1 #3 Solution: Let { x , x 1 , x 2 , x 3 } = {- 1 , 1 , 3 , 4 } . We find the four corresponding Lagrange polynomials to be ` = ( x- 1)( x- 3)( x- 4) (- 1- 1)(- 1- 3)(- 1- 4) ` 1 = ( x +1)( x- 3)( x- 4) (1+1)(1- 3)(1- 4) ` 2 = ( x +1)( x- 1)( x- 4) (3+1)(3- 1)(3- 4) ` 3 = ( x +1)( x- 1)( x- 3) (4+1)(4- 1)(4- 3)-2-1 1 2 3 4 5-3-2-1 1 2 3 4 Lagrange Polynomials The key property your graph should show is that ` i ( x j ) = 1 i = j i 6 = j 3. [1pt] Section 4.1 #4 Solution: To verify that the two polynomials interpolate the data, just note that for all four points p ( x i ) = q ( x i ) = y i . The “Existance of Polynomial Interpolation Theorem” guarantees that for four data points there is a unique polynomial of degree three or less which interpolates the data. q is of degree four so the theorem does not apply. 4. [1pt] Section 4.1 #13 Solution: 1 (a)-1 1-1 2-1-1-7 2-1 1 3 5 3 1 1 p ( x ) =- 1 + 0 x + 0 x ( x- 1) + 1 x ( x- 1)( x- 2) + 0 x ( x- 1)( x- 2)( x + 1) (b) 1 2 3 6 2-2-1 7/5 1/5 4-4 1/2-19/10-7/10 5 2 6 13/14 99/70 37/70 p ( x ) = 2 + 2( x- 1) + 1 5 ( x- 1)( x- 3)- 7 10 ( x- 1)( x- 3)( x + 2) + 37 70 ( x- 1)( x- 3)( x + 2)( x- 4) 5. [1pt] Section 4.1 #18 Solution: 2 1 1-1 3 5 2 1 2 6-1-3-2 5-183-63-31-7-1 p ( x ) = 2- 1 x + 1 x ( x- 1)- 2 x ( x- 1)( x- 3)- 1 x ( x- 1)( x- 3)( x- 2) Rearanging for fast evaluation we find, p ( x ) = 2 + x (- 1 + ( x- 1)(1 + ( x- 3)(- 2 + ( x- 2)(- 1)))) 6. [1pt] Section 4.1 #40 Solution: To create a divided difference table with n + 1 points, one will need to perform arithmatic to put data in in n columns: one for f [ , ], one for f [ , , ], etc. The first column will contain n enteries, the second n- 1, and so on. The total number of enteries to be filled in will be n + ( n- 1) + ( n- 2) + . . . + 2 + 1 = n ( n + 1) 2 For each entry one must perform two subtractions, one division, and no multiplications. Therefore, the total number of subtractions is n ( n + 1). The total number of divisions is n ( n +1) 2 . 7. [1pt] Section 4.1 #42 Solution: Given a polynomial p ( x ) of degree n and n + 2 numbers x i , imagine generating the data points [ x i , p ( x i )].We know that we can find a Newton polynomial of these data points using the divided difference formula. p n +1 ( x ) = p [ x ] + p [ x , x 2 ]( x- x ) + . . . + p [ x , . . . , x n +1 ]( x- x )( x- x 1 ) . . . ( x- x n ) Note that the above Newton polynomial is of degree less than or equal to n + 1. We know that the degree n polynomial p also interpolates the data. By “Existance of Polynomial Interpolation Theorem”, there is a unique polynomial of degree n + 1 or less that interpolate that data points. Hence, p and p n +1 are equal....
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## This note was uploaded on 09/15/2008 for the course CS 257 taught by Professor Thomaskerkhoven during the Fall '05 term at University of Illinois at Urbana–Champaign.

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hw6_solutions - CS 257 Numerical Methods - Homework 6...

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