231.12.3.19 angle between vectors

[karush]

$\tiny{231.12.3.19}$
$\textsf{Given $v=-7i-j$ and $w=-i-7j$}\\$
$\textsf{find the angle between v and w}\\$
$\displaystyle
\frac{\left(-7, -1, 0\right)\cdot\left(-1, -7, 0\right)}
{\sqrt{50}\cdot \sqrt{50}}
=\frac{14}{50}\approx 0.28$
$\arccos(0.28)\approx 73.74^o$
$\textit{not sure if this is the answer but is cross product also solved with a matrix?}$

 

[Greg]

Your answer looks good to me.

What are the zeros for? Cross-product is 3-d. What you have here appears to be 2-d and you've used the dot product.

 

[karush]

I see a lot of vector notation with z=0
thot i would join the club.;)

 

[HallsofIvy]

It is best to use the same notation as the problem. I would have written $\frac{(-7i- j)\cdot (-i- 7j)}{\sqrt{50}\sqrt{50}}$