Thursday, February 7, 2008


Hey guys!
So, we already learned how to add matrices and multiply matrices by scalars. Today we learned how to solve MATRIX MULTIPLICATION.


- it is the operation of multiplying a matrix with either a scalar or other matrix.

Matrix multiplication is not commutative except in special cases.

Commutative law is when numbers can be added or multiplied in any order

Here is how to do it:

We are asked to multiply matrix A and matrix B.

So first, we have to CHECK the DIMENSIONS. If middle numbers aren't matching we cant multiply them. The middle numbers have to be matching to be able to calculate it. If they are not matching it is not possible to calculate it.
Since they both have 2x2 dimension we dont really need to check the middle number.

-You multiply the rows of matrix A by the columns of matrix B.
-Take the first row of A and the first column of B.
-Then, multiply the first entries and the second entries and then add the two products.
-The sum is one entry of the product matrix AB.
-You just do the same thing when you take the second row of A and the second column of B.

Here are other examples that we did in class today:

I hope you find my explanation and examples understandable. If you still have problems in this topic please see

The next scribe is Chris29.


elaineee said...

I really love the way you used those pictures to explain how to solve the problems! great job! :)

Lani said...

Hi Charmel,

I totally agree with Elaineee; the images really enhance the explanation. The additional Purple Math link helped with my understanding too! Thanks!