got it, thanks LanX

fyi, I googled it a lot before asking this particular question. "when are 2 vectors / 2 planes are parallel". What I got was lots and lots of the ratio test but not mentioning at all any of the edge cases. I finally asked here for a clarification : https://math.stackexchange.com/questions/3925150/find-if-2-planes-are-parallel-or-not, the answeree has a huge XP but fails to complete their answer with common sense question.

In reply to Re^4: The intersection of M hyperplanes (Ndim) by bliako
in thread The intersection of M hyperplanes (Ndim) by bliako

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