# Math Help - Vector magnitude addition

1. ## Vector magnitude addition

I hope this is the right forum.

The question is about vector addition. Suppose you have two vectors:

$u=<1,1,1>, v=<-1,2.1>$

The magnitudes are:

$||u+v||=\sqrt{13}$ and

$||u|| + ||v|| = \sqrt{3} + \sqrt{6}$

But the answer in the back of the book is showing $\sqrt{13}$ for both? Where am I going wrong?

2. ## Re: Vector magnitude addition

I agree with your answers. Moreover, in a euclidean space, $\|u+v\| = \|u\|+\|v\|$ iff u and v are proportional, which is not the case here. There is probably a typo in the book.

3. ## Re: Vector magnitude addition

Originally Posted by M.R
But the answer in the back of the book is showing $\sqrt{13}$ for both? Where am I going wrong?
I think you are correct, assuming I am reading your work correctly.

$\mathbf u = \langle1,1,1\rangle, \mathbf v = \langle-1,2,1\rangle$

$\|\mathbf u + \mathbf v\| = \|\langle0,3,2\rangle\| = \sqrt{13}$

$\|\mathbf u\| + \|\mathbf v\| = \sqrt3 + \sqrt6$

$\sqrt{13}\neq\sqrt3+\sqrt6$

4. ## Re: Vector magnitude addition

Thank you both for the verification.

BTW, where is the thanks button?

5. ## Re: Vector magnitude addition

Apparently. in order for the Thanks button to be visible, the mouse pointer has to be located on the post.

6. ## Re: Vector magnitude addition

Yes, it appears on the bottom right hand corner. I liked the old system better though.