1. [25 points] For the matrix A below, find a basis for the null space of A, a basis for the row space of A,
a basis for the column space of A, the rank of A, and the nullity of A. The reduced row echelon form
of A is the matrix R given below.
A =
2 2 6 6 −2 −18
−5 3 9 1 37 5
0 −3 −9 −6 −12 15
1 5 15 11 15 −29
R =
1 0 0 1 −5 −4
0 1 3 2 4 −5
0 0 0 0 0 0
0 0 0 0 0 0
Solution: To find a basis for the null space, you need to solve the system of linear equations A~x = ~0,
or equivalently, R~x = ~0. Parameterizing the solutions to this equation, and writing the answer in
expanded form, produces
x1
x2
x3
x4
x5
x6
= α1 ·
0
−3
1
0
0
0
+ α2 ·
−1
−2
0
1
0
0
+ α3 ·
5
−4
0
0
1
0
+ α4 ·
4
+5
0
0
0
1
so
4
5
0
0
0
1
,
5
−4
0
0
1
0
,
−1
−2
0
1
0
0
,
0
−3
1
0
0
0
is a basis for the null space of A. The nullity is the number of vectors in this basis, namely 4.
A basis for the row space can be found by taking the nonzero rows of R:
{[ 1, 0, 0, 1, −5, −4 ] , [ 0, 1, 3, 2, 4, −5 ]}
A basis for the column space can be found by taking the columns of A which have pivots in them, so
2
3
−3
5
,
2
−5
0
1
is a basis for the column space of A.
Lastly, the rank of A is the number of vectors in a basis for the row space (or column space)
of A, so the rank of A is 2.
Grading: +5 points for each of: finding a basis for the null space, a basis for the row space, a
basis for the column space, the nullity, and the rank. Grading for common mistakes: −3 points for
forgetting a variable in the parameterization; −3 points for choosing columns of R for the column
space of A; −3 points for choosing rows from A for the row space of A; −3 points for choosing the
non-pivot columns of A for the null space of A.
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