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Unformatted text preview: Math 235 Assignment 7 Solutions 1. For each quadratic form Q ( vectorx ), determine the corresponding symmetric matrix A . By diagonalizing A , express Q ( vectorx ) in diagonal form and give an orthogonal matrix that diago nalizes A . Classify each quadratic form. a) Q ( x, y ) = 7 x 2 + 12 xy + 12 y 2 . Solution: We have A = bracketleftbigg 7 6 6 12 bracketrightbigg so A I = bracketleftbigg 7 6 6 12 bracketrightbigg . The characteristic equation is 0 = det( A I ) = 2 19 + 48 = ( 3)( 16) . The roots are 3 and 16, so these are the eigenvectors of A . For = 3 we get A 3 I = bracketleftbigg 4 6 6 9 bracketrightbigg bracketleftbigg 2 3 0 0 bracketrightbigg vector z 1 = bracketleftbigg 3 2 bracketrightbigg . For = 16 we get A 16 I = bracketleftbigg 9 6 6 4 bracketrightbigg bracketleftbigg 3 2 bracketrightbigg vector z 2 = bracketleftbigg 2 3 bracketrightbigg . Normalizing the vectors, we get that the orthogonal matrix which diagonalizes A is P = bracketleftbigg 3 / 13 2 / 13 2 / 13 3 / 13 bracketrightbigg , and Q = 3 x 2 1 + 16 y 2 1 , where bracketleftbigg x 1 y 1 bracketrightbigg = P T bracketleftbigg x y bracketrightbigg . Since all the eigenvalues of A are positive, it follows that Q ( x, y ) is positive definite. b) Q ( x, y ) = x 2 + 6 xy 7 y 2 Solution: We have A = bracketleftbigg 1 3 3 7 bracketrightbigg so A I = bracketleftbigg 1 3 3 7 bracketrightbigg . The characteristic equation is 0 = det( A I ) = 2 + 6 16 = ( 2)( + 8) . The roots are 2 and 8, so these are the eigenvectors of A . For = 2 we get A 2 I = bracketleftbigg 1 3 3 9 bracketrightbigg bracketleftbigg 1 3 bracketrightbigg vector z 1 = bracketleftbigg 3 1 bracketrightbigg . For = 8 we get A + 8 I = bracketleftbigg 9 3 3 1 bracketrightbigg bracketleftbigg 3 1 0 0 bracketrightbigg vector z 2 = bracketleftbigg 1 3 bracketrightbigg . Normalizing the vectors, we get that the orthogonal matrix which diagonalizes A is P = bracketleftbigg 3 / 10 1 / 10 1 / 10 3 / 10 bracketrightbigg , and Q = 2 x 2 1 8 y 2 1 , where bracketleftbigg x 1 y 1 bracketrightbigg = P T bracketleftbigg x y bracketrightbigg ....
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This note was uploaded on 10/12/2010 for the course MATH 235 taught by Professor Celmin during the Fall '08 term at Waterloo.
 Fall '08
 CELMIN
 Math

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