DOT Product

**kwtolley** · July 20th 2006, 07:24 AM

The dot product [3 1 4]dot 2
-1
5 is.

My answer is 25, but I'm not sure. Thanks for any help given.

**CaptainBlack** · July 20th 2006, 09:09 AM

Originally Posted by kwtolley

The dot product [3 1 4]dot 2
-1
5 is.

My answer is 25, but I'm not sure. Thanks for any help given.

The dot product of two vectors $[x_1,\ x_2,\ x_3]$ and $[x_1,\ x_2,\ x_3]$ is:

$ \bold{x}\cdot \bold{y}=\sum_{i=1}^3 x_i y_i = x_1y_1+x_2y_2+x_3y_3 $ .

In this case we have $\bold{x}=[3,1,4]$ and $\bold{y}=[2,-1,5]$ , so:

$ \bold{x}\cdot \bold{y}= 3\times 2+1\times (-5)+4\times 5=21 $ .

RonL

**kwtolley** · July 20th 2006, 09:19 AM

I really needed help on that one. I was way off on seting it up. thanks again for checking

**topsquark** · July 20th 2006, 11:43 AM

Originally Posted by CaptainBlack

In this case we have $\bold{x}=[3,1,4]$ and $\bold{y}=[2,-1,5]$ , so:

$ \bold{x}\cdot \bold{y}= 3\times 2+1\times (-5)+4\times 5=21 $ .

RonL

Hold on! There was a typo:

The last line should read:
$\bold{x}\cdot \bold{y}= 3\times 2+1\times (-1)+4\times 5=25 $ .

So you were correct, ktwolley.

-Dan

**CaptainBlack** · July 20th 2006, 12:21 PM

Originally Posted by topsquark

Hold on! There was a typo:

The last line should read:
$\bold{x}\cdot \bold{y}= 3\times 2+1\times (-1)+4\times 5=25 $ .

So you were correct, ktwolley.

-Dan

Opps, there you go, you should always check what a helper writes.
Hopefully they provide enough information that they have in fact done
what they say they are doing

RonL

**kwtolley** · July 20th 2006, 07:59 PM

Thanks both of you. I check and recheck myself and other, but sometimes the problems are hard. Thanks again you guys here are a big help.

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