Matrix Multiplication

[kev77]

Is there a way to calculate a specific entry without doing the whole matrix multiplication by hand?

I ask this because clearly, 3points aren't worth the time it would take to do the whole matrix multiplication.

 

[Dr.Peterson]

Is there a way to calculate a specific entry without doing the whole matrix multiplication by hand?

I ask this because clearly, 3points aren't worth the time it would take to do the whole matrix multiplication.

View attachment 32132

Think about what entries of AB you need to know in order to find [imath]d_{2,4}[/imath]. (Or, what entries of BC you need.) Then find just that part.

You might even sketch the matrices involved without filling in any numbers, mark which numbers you need, and fill those in.

If you need more help, please show us whatever you have tried, so we can see if you're going in the right direction.

 

[pka]

SEE HERE what is [imath]d_{2,4}=~?[/imath]

 

[Dr.Peterson]

Is there a way to calculate a specific entry without doing the whole matrix multiplication by hand?

Here are the only parts I needed to use in order to calculate that one entry, using the method I suggested:

​

Do you see why?