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Unformatted text preview: Math 24 Winter 2010 Sample Solutions to the Midterm (1.) (a.) Find a basis { v 1 ,v 2 } for the plane P in R 3 with equation 3 x + 2 y z = 0. We can take any two noncollinear vectors in the plane, for instance v 1 = (1 , , 3) and v 2 = (0 , 1 , 2). (b.) You know from multivariable calculus that the vector v 3 = (3 , 2 , 1) is perpendicular to the plane P . Therefore = { v 1 ,v 2 ,v 3 } is linearly independent, and forms an ordered basis for R 3 . Let T : R 3 R 3 be the perpendicular projection onto the plane P . In other words, T ( v ) is the perpendicular projection of v onto P . What is [ T ] ? As v 1 and v 2 are in the plane, T ( v 1 ) = v 1 and T ( v 2 ) = v 2 . As v 3 is a vector perpendicular to the plane, its projection is the origin, T ( v 3 ) = 0. Therefore [ T ( v 1 )] = [ v 1 ] = 1 , [ T ( v 2 )] = [ v 2 ] = 1 , [ T ( v 3 )] = [0] = , [ T ] = 1 0 0 0 1 0 0 0 0 . (c.) Let be the standard ordered basis for R 3 . Find the change of coordinate matrices Q that changes coordinates into coordinates, and Q that changes coordinates into coordinates. Do not use matrix inversion (if you know how to invert matrices) to do this problem. Find each matrix by explicitly computing the coordinates of the appropriate vectors in the appropriate bases. If you wish, you can check your work by verifying that Q Q = I . Q = 1 0 3 0 1 2 3 2 1 , the matrix whose columns are the (standard) coordinates of the vectors in . To find the coordinates of the vectors in (the standard basis vectors) we need to solve the vector equations: (1 , , 0) = a (1 , , 3) + b (0 , 1 , 2) + c (3 , 2 , 1) (0 , 1 , 0) = a (1 , , 3) + b (0 , 1 , 2) + c (3 , 2 , 1) (0 , , 1) = a (1 , , 3) + b (0 , 1 , 2) + c (3 , 2 , 1) When we do this, we get (1 , , 0) = 5 14 (1 , , 3) 6 14 (0 , 1 , 2) + 3 14 (3 , 2 , 1) (0 , 1 , 0) = 6 14 (1 , , 3) + 10 14 (0 , 1 , 2) + 2 14 (3 , 2 , 1) 1 (0 , , 1) = 3 14 (1 , , 3) + 2 14 (0 , 1 , 2) 1 14 (3 , 2 , 1), that is, [(1 , , 0) = 5 14 6 14 3 14 [(0 , 1 , 0)] =  6 14 10 14 2 14 [(0 , , 1)] = 3 14 2 14 1 14 . Now, using these coordinates as the columns of Q , we have Q = 5 14 6 14 3 14 6 14 10 14 2 14 3 14 2 14 1 14 . (d.) Find the matrix of T in the standard basis, [ T ] . If you want to use the result of part (b) but were not able to do part (b), you may pretend [ T ] = 1 0 0 0 1 0 0 0 2 . This is not the correct answer to part (b)....
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This note was uploaded on 02/22/2012 for the course EE 441 taught by Professor Neely during the Spring '08 term at USC.
 Spring '08
 Neely

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