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Let a be the direction vector <a1, a2, a3>
Let b be the direction vector <b1, b2, b3>
Cross Product (final answer is a vector):
a × b = <a1, a2, a3> × <b1, b2, b3> =
<(a2)(b3) - (a3)(b2),
(a3)(b1) - (a1)(b3),
(a1)(b2) - (a2)(b1)>
-or-
a × b = (|a||b|sinθ) n, where θ is the angle between the two vectors
and n is the unit vector normal (perpendicular) to
the plane created by a and b.