Matrix Help
 

I feel stupid for being a senior and not being able to figure this out:

Multiply the matrices AD

A
[ 1 0
-2 4
5 -1 ]

D
[ 5 -2 -1
4 3 1 ]

A
[ 1 0
-2 4
5 -1 ]

D
[ 5 -2 -1
4 3 1 ]

The first step in a problem like this is to make sure the matrices can actually be multiplied. Here we have a 3x2 * 2x3. The quick check is to make sure the inside numbers are the same. If they are, then the matrices can be multiplied.

The basic setup for this problem goes like this:

[ a b
c d
e f ]

*
[ g h i
j k l ]

= [ ag+bj ah+ak ai+al
cg+dj ch+dk ci+cl
eg+fj eh+fk ei+el ]

So, take it from there!

I'm very right-brained. All I see is a bunch of letters. But thanks for taking the time to help.

firebirdta you've made a few typos, the last column in A*D should be:

[ai + bl, ci + dl, ei + fl]

essendoubleop: How familiar are you with dot products?

The first entry of A*D (upper left) is the dot product of the first row in A with the first column in D.

The second entry (upper middle) is the dot product of the first row of A with the second column in D.

The third entry (upper right) is the dot product of the first row of A with the third column of D

The fourth entry (bottom left) is the dot product of the second row of A with the first column of D.

et cetera.

Does that help you?

mathmo, you are correct, I typed it in a different way, but the formatting was wrong, so I quickly retyped it and didn't check it very carefully. Good checking!