Using cross product to find angle between two vectors

[yayscience]

Homework Statement

Find the angle between
\begin{align*}<br /> \vec{A} = 10\hat{y} + 2\hat{z} \\<br /> and \\<br /> \vec{B} = -4\hat{y}+0.5\hat{z} <br /> \end{align*}
using the cross product.

The answer is given to be 161.5 degrees.

Homework Equations

<br /> \left| \vec{A} \times \vec{B} \right| = \left| \vec{A} \right| \left| \vec{B} \right|sin(\theta)<br /> <br />

The Attempt at a Solution

<br /> \left| \vec{A} \times \vec{B} \right| = \left|<br /> \begin{array}{ccc}<br /> \hat{x} &amp; \hat{y} &amp; \hat{z} \\<br /> 0 &amp; 10 &amp; 2 \\<br /> 0 &amp; -4 &amp; 0.5<br /> \end{array} \right| = \left| 13\hat{x} \right| = 13

The magnitude of A cross B is 13.

Next we find the magnitude of vectors A and B:
\left| \vec{A} \right| = \sqrt{10^2+2^2} = \sqrt{104} = 10.198039
and
\left| \vec{B} \right| = \sqrt{(-4)^2+(\frac{1}{2})^2} = \sqrt{16.25} = 4.0311289

multiplying the previous two answers we get:
41.109609

So now we should have:
\frac{13}{41.109609} = sin(\theta)

Solving for theta, we get:
18.434951 degrees.


This is frustrating: 180-18.434951 = the correct answer. I'm not quite sure where I'm going wrong here.

I must be making the same mistake repeatedly. Another problem was the same thing, but with the numbers changed, and I also got the 180-{the answer I was getting} = {the correct answer}, but when I tried the example using the SAME methodology, I got the correct answer.

Can someone please share some relevant wisdom in my direction?

 

[ehild]

sin(alpha)=sin(180-alpha) Plot the two vectors and you will see what angle they enclose.ehild

 

[I like Serena]

You might use the sign of the inner dot product to see which angle you have.

 

[yayscience]

I can plot them, and I can see the angle, but I'm interested in calculating the angle.
When I use the dot product I get the correct result, but I cannot see where my mistake is while using the cross product.

 

[ehild]

There is no mistake, you get the sine of the angle, but there are two angles between 0 and pi with the same sine.

ehild

 

[yayscience]

Oh wow; I didn't even consider that the answer wasn't unique.
Thanks!