1. 1.4 The Matrix Equation Ax = b
DEFINITION
The product of A and x, is the linear combination of the columns of A using the corresponding entries in x as weights; that is,

2. EXAMPLE1

3. EXAMPLE 2
v1, v2, v3 Rm ,
EXAMPLE 3
a matrix equation

4. Ex1:

5. THEOREM 3
The matrix equation Ax=b has the same
solution set as the vector equation
x1a1+ x2a2+ … + xnan = b,
which, in turn, has the same solution set as
the system of linear equations whose
augmented matrix is [a1 a2 … an b].

6. Existence of Solution
The equation Ax = b has a solution if and only if b is a linear combination of the columns of A.

7. Ex2: Solve the matrices equations:

8. THEOREM 4
The following statements are logically equivalent.
a. For each b in Rm, the equation Ax = b has a solution.
b. Each b in Rm is a linear combination of the column of A.
c. The columns of A span Rm.
d. A has a pivot position in every row.

9. EXAMPLE 4 No. For there are two pivot positions.
is the equation Ax=b consistent
for all possible b1, b2, b3 ?
No. For there are two pivot positions.

10. Ex3: Yes. For A has a pivot position in every row
is the equation Ax=b consistent for all possible b1, b2, b3 ?
Yes. For A has a pivot position in every row

11. Ex4: . No. For there are two pivot positions.
is the equation Ax=b consistent for all possible b1, b2, b3 ?
No. For there are two pivot positions.

12. Computation of Ax (Row-Vector Rule for Computing Ax)
The ith entry in Ax is the sum of the products of corresponding entries from row i of A and from the vector x.

13. EXAMPLE5

14. Identity matrix I 单位矩阵
I x = x

15. Properties of the Matrix-Vector Product Ax
THEROM 5 If A is an m×n matrix, u and v are vectors in Rn, then
a. A( u + v ) = Au+Av; b. A(cu)=c(Au).