1. Dimensions matching Rows times Columns
4.3 Multiplying Matrices
Dimensions matching
Rows times Columns

2. The end of one matrix and the begin of the other matrix must match up
When multiplying two matrices (A and B) the dimension of A must end and the dimension of B must start with the same number.

3. The resulting matrix dimension come from multiplying
The answer comes from the beginning of one matrix and the end of the other.
Let matrix C dimensions be 3 X 2 and matrix D dimensions be 2 X 5. When multiplying the matrix C times matrix D, the answer is a matrix with a dimension of 3 X 5.
answers dimensions
3 X 2 times 2 X 5
must have

4. Order matters One of the big rules in multiplying matrices.
A * B ≠ B * A
even if the dimensions match.

5. Rows times Columns
Ever element in a row is added to the multiplication of every element in the column.
A B = AB

6. Ok, so that is how easy it is.
Yes and No.
Not all matrices are 2 X 2.
Same way row element times column element, then added to the next row element times column element, till out of elements, then next column

7. Rows times columns
Take you time

8. Rows times columns
Take you time =

9. Is order really important
Not the same.

10. Find
A(B + C)

11. Find
A(B + C)

12. Find
A(B + C)

13. Find
A(B + C)

14. Find
AB + AC
Same as
A(B + C)