In this pic
http://flic.kr/p/bCFYK7
do you see where on the second row they multiply [-2 1 0] by 4 and get 0? I don't see how they get 0.

Question

In this pic 
http://flic.kr/p/bCFYK7
do you see where on the second row they multiply [-2 1 0] by 4 and get 0?  I don't see how they get 0.

anonymous · Answer

It's because of how matrix multiplication is done. You don't multiply straight across.

anonymous · Answer

Are you familiar with the dot product?
To get the 1st entry of the result, it's the dot product of row 1* column 1.

amistre64 · Answer

you state: "do you see where on the second row they multiply [-2 1 0] by 4 and get 0?"

the 2nd row is not multiplied by 4

amistre64 · Answer

-2a+1b+0c = 4  when a=2
                                   b=4
                                   c=-2

anonymous · Answer

Thanks, I got it now.

anonymous · Answer

Ah, good. =D