A cross B equals zero is a test for orthogonal cases.
If AxB=0 then A is perpendicular to B.
Uh...is that a question?
No, it's not. It is the dot product. If A and B are vectors, the [itex]A\cdot B= 0[/itex] if and only if they are orthogonal (perpendicular).
You are confusing this with the fact that if A and B are any two (three dimensional) vectors, then [itex]A\times B[/itex] is perpendicular to both A and B.
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