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The reduced row echelon form of the matrix −1 1 1 1 is the identity matrix, so the vectors form a basis

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MA 1111: Linear Algebra I Tutorial problems, October 19, 2018

1. The reduced row echelon form of the matrix

−1 1

1 1

is the identity matrix, so the vectors form a basis.

2.There are more than two vectors, so they are not linearly independent, and do not form a basis. The first and the third form a basis, hence span R2, hence the three also span R2.

3. There are more than three vectors, so they are not linearly independent, and do not form a basis. The matrix formed by the first, the second, and the last vector has the determinant 9, so is invertible, so its reduced row echelon form is the identity matrix, so those three vectors spanR3, hence all the five vectors span R3.

4. The matrix formed by these vectors has the determinant 2, so is invertible, so its reduced row echelon form is the identity matrix, so those three vectors form a basis.

5. (a) Suppose thatc1(u−2w) +c2(v+w) +c3w=0. Since

c1(u−2w) +c2(v+w) +c3w=c1u+c2v+ (−2c1+c2+c3)w,

and u,v, ware linearly independent, we have c1 =c2 = −2c1+c2+c3 =0, from which we deducec1 =c2 =c3 =0.

(b) Suppose that we want to find coefficients c1,c2, c3 that give

c1(u−2w) +c2(v+w) +c3w=b.

Since

c1(u−2w) +c2(v+w) +c3w=c1u+c2v+ (−2c1+c2+c3)w,

we may instead solvec1u+c2v+ (−2c1+c2+c3)w=b. By assumption, the system of vectors u, v, w is complete, so we can write a1u+a2v+a3w =b, and it is enough to setc1 =a1, c2 =a2, and c3 =2c1−c2+a3.

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