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Unformatted text preview: Math 235: Linear Algebra HW 5 Problem 2.6.2 Problem 2.6.2 Let 1 = (1 , 1 , 2 , 1) , 2 = (3 , , 4 , 1) , 3 = ( 1 , 2 , 5 , 2) and Let = (4 , 5 , 9 , 7) , = (3 , 1 , 4 , 4) , = ( 1 , 1 , , 1) (a) Which of the vectors , , are in the subspace of R 4 spanned by the i ? Put the i into a matrix and rowreduce. 1 1 1 1 3 4 1 1 2 5 2 1 0 0 3 / 13 0 1 0 14 / 13 0 0 1 1 / 13 So we see that anything the subspace spanned by the i are of the form ( a,b,c, 1 / 13( 3 a + 14 b c )). So we proceed to check , and . (4 , 5 , 9 , 1 / 13( 12 70 9)) = (4 , 5 , 9 , 7) = . (3 , 1 , 4 , 1 / 13( 9 + 14 + 4)) = (3 , 1 , 4 , 9 / 13) 6 = . ( 1 , 1 , , 1 / 13(3 + 14)) = ( 1 , 1 , , 17 / 13) 6 = . So is the only vector in the space generated by the i . (a) (b) Which of the vectors , , are in the subspace of C 4 spanned by the i ? For the exact same reason, is the only vector in the space generated by the i . (b) (c) Does this suggest a theorem? Yes. Let F and K be fields such that F K . Let { 1 ,..., m } be a set of vectors all of whose components are in F . Let v be any vector whose components are all elements of F then: v is in the Fvector space generated by the i if and only if v is in the Kvector space generated by the i . (c) Page 1 of 7 Math 235: Linear Algebra HW 5 Problem 2.6.3 Problem 2.6.3 Consider the vectors in R 4 defined by 1 = ( 1 , , 1 , 2) , 2 = (3 , 4 , 2 , 5) , 3 = (1 , 4 , , 9) . Find a system of homogeneous linear equations for which the space of solutions is exactly the subspace of R 4 spanned by the three given vectors. First: row reduce the matrix with the i as its rows:  1 0 1 2 3 4 2 5 1 4 9 1 0 1 2 0 1 1 / 4 11 / 4 0 0 So the system of equations we get is x 1 x 3 2 x 4 = 0 x 2 + 1 / 4 x 3 + 11 / 4 x 4 = 0 Problem 2.6.5 Give an explicit description of the type (225) for the vectors = ( b 1 ,b 2 ,b 3 ,b 4 ,b 5 ) R 5 which are a linear combination of the vectors 1 = (1 , , 2 , 1 , 1) , 2 = ( 1 , 2 , 4 , 2 , 0) , 3 = (2 , 1 , 5 , 2 , 1) , 4 = (2 , 1 , 3 , 5 , 2) ....
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 Spring '10
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