My post came back to me and I realized it doesn't allow for negative arguments to arccos. Does Perl have a signum function? The wiki says you can use sgn()
(http://webwork.maa.org/wiki/Available_Functions)
So then:
arccos(sgn($u . $v) * sqrt((($u . $v)**2)/(($u . $u)*($v . $v)))
WeBWorK Problems
Can't take acos of 1 or -1?
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