(b) If a multiple of one row of A is added to another row to produce B then det(b) =det(a).

Transcription

1 .(5pts) Let B = 5 5. Compute det(b). (a) (b) (c) 6 (d) (e) 6.(5pts) Determine which statement is not always true for n n matrices A and B. (a) If two rows of A are interchanged to produce B, then det(b) = det(a). (b) If a multiple of one row of A is added to another row to produce B then det(b) =det(a). (c) det(a + B) = det(a) + det(b) (d) If one row of A is multiplied by k to produce B, then det(b) =k det(a). (e) det(ab) = det(a) det(b).(5pts) Determine which set is not a subspace of P n, the space of polynomials of degree at most n. (a) The set of p(t) in P n such that p() = p(). (b) The set of p(t) in P n such that p( t) = p(t). (c) The set of p(t) in P n such that p() =. (d) The set of p(t) in P n of the form p(t) =at + at where a is in R. (e) The set of p(t) in P n of the form p(t) =t + at where a is in R..(5pts) Determine which statement is always true. (a) The rows of a matrix A containing the pivot positions form a basis for Row A. (b) If H = Span {b,...,b n }, then {b,...,b n } is a basis for H. (c) A linearly independent set in a subspace H is a basis for H. (d) If a finite set S of nonzero vectors spans a vector space V, then some subset of S is a basis of V. (e) A basis is a spanning set that is as large as possible.

2 5.(5pts) The set B = { t, t t,t t,t } is a basis for P, the space of polynomials of degree at most. Find the coordinate vector of p(t) = t+5t t relative to the basis B. (a) (b) (c) 6 5 (d) 6 5 (e) (5pts) Let A = Find a basis for Row (A). (a) (,,, ), (,,, ) (b) (,,, ), (,,, 6) (c) (,,, ), (,,, ), (,,, ) (d) (,,, ), (,,, ), (,,, ) (e) (,, 5, 6), (,,, ).(5pts) Let B =, and C = 8 the change-of-coordinates matrix from B to C. 5 (a) (b) (c), (d) 5 be bases for R. Find P C B, (e) (5pts) The eigenvalues of a certain matrix A are,, and. Determine which statement is true. (a) Ax = b is inconsistent for some b in R. (b) Ax = has only the trivial solution. (c) rank (A) = (d) A is not diagonalizable. (e) det(a) = 9.(5pts) Determine which vector is an eigenvector of the matrix 5.

3 (a) 5 (b) 5 (c) 5 (d) 5 (e) 5.(5pts) Let A =, and let B = for the transformation x! Ax. 6 5 (a) (b) (c) 8 6 5, be a basis of R. Find the B-matrix (d) 5 (e) (pts) Let A = 5. Use cofactors to compute adj (A)..(pts) Let A = 5. Find an invertible matrix P and a diagonal matrix D such that A = PDP..(pts) Find the complex eigenvalues and eigenvectors of the matrix A =. The transformation x! Ax is the composition of a rotation and a scaling. Give the angle of the rotation and the scale factor r.

4 Solutions =( )( 5) + (6 ( 5)) = =. All but det(a + B) = det(a) + det(b) are standard rules for manipulating determinants. Note det 6= det + det. Since + 6=, the set of p(t) in Pn such that p() = is not closed under addition. The others are. Similarly, all but the set of p(t) in P n such that p() = are closed under scalar multiplication.. (d) is a theorem in the book and so is always true. The others are approximates statements of results from the book but none of the others is quite right. 5. p(t) =( t)+t+5t so t =( t)+(t t )+8t t =( t)+(t t )+8(t t )+6t 6. Reduce to echelon form The notation for the column vectors in the book uses parentheses so (a) is 6 correct.. Write = c + c 5. Either solve or by inspection c = c = so first column is. (b) is the only choice with this column but if you want ot work out the second column solve 8 = c + c 5 to get c =,c =.

5 8. Since is an eigenvalue, the null space has dimension at least and since there are two more distinct eigenvalues, all three eigenspaces have dimension exactly. Hence the rank is as is the dimension of the column space. Therefore (a) is true. To see the others note that the determinant is ( ) =. The matrix is diagonalizable since there are three distinct eigenvalues a 9. Since two rows are equal, the matrix has a non-trivial null space and either by row reduction or inspection 5 is an eigenvector with eigenvalue. This is not a good multiple choice problem since the quickest way to get the answer is to evaluate the matrix on the answers. To work out the characteristic polynomial, find the root(s) and then the eigenvectors is a waste a time.. 9 = =9 +( 5) 5 so the first column is = =6 +( 9) 5 so the first column is The B matrix is There are 9 cofactors C, =( ) + det = C, =( ) + det = C, =( ) + det = C, =( ) + det = C, =( ) + det = C, =( ) + det = C, =( ) + det =

6 C, =( ) + det = C, =( ) + det = So the i, j rmth entry in the adjunct of A is C j,i so the adjunct is 5. The characteristic polynomial is det and since this matrix is upper triangular, the determinant is ( )( )( ) so the eigenvalues are, and and all eigenspaces are dimensional. The eigenspace for is the null space for 5. One vector in this null space is 5 so it is a basis. The eigenspace for is the null space for 5. One vector in this null space is 5 so it is a basis. The eigenspace for is the null space for 5. One vector in this null space is 8 65 so it is a basis. Hence P = 8 65 and D = 5. You need do no further work since the results in the book tell you that these choices will work out.. The characteristic equation is det are = ± p 6 =± i. The eigenspace for + i is the null space for eigenspace. =( ) += + 8. The roots i i and i is a basis for the

7 The eigenspace for the conjugate i i is the null space for and i i is a basis for the eigenspace. To work out the angle and the scaling, results in the book say that the scaling is by = p + = p and the angle of rotation counterclockwise is where + i = p (cos + i sin ). Hence is 5 degrees or radians.. (a) (b) (c) ( ) (e). (a) (b) ( ) (d) (e). (a) (b) ( ) (d) (e). (a) (b) (c) ( ) (e) 5. (a) ( ) (c) (d) (e) 6. ( ) (b) (c) (d) (e). (a) ( ) (c) (d) (e) 8. ( ) (b) (c) (d) (e) 9. (a) (b) ( ) (d) (e). (a) (b) (c) (d) ( )