10.5#18

For this problem, I found the cross product of the two vectors <1,2,0> and <0,1,1> to find the vector components of the plane. Then, I used |a*b|=|a||b|cos(theta) where a is <2,-1,1> and b was <0,0,1>. However, I seem to be getting the problem wrong. I tried <0,1,0>, since perhaps I was confusing the vertical axis, but that still seemed to be wrong.


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Uhm....in the first place based on your answer it looks like your calculator is using degrees instead of radians, since 65.9 radians is about eight or times around the circle 

(This one example of what is called a "smell test", or a "do-my-numbers-make-any-damn-sense test". Learning how to come up with these tests and answer them is an IMPORTANT skill.)  

In the second place the formula for dot product is

u.v = ||u|| ||v|| cos(𝜽)  

with no absolute value, not that this is important for this particular problem. Looks like you computed the cross product and the dot product correctly.