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WikiPedia: Trực giao http://mathworld.wolfram.com/Orthogonal.html http://www.faqs.org/docs/artu/ch04s02.html https://books.google.com/books?id=bj-Lu6zjWbEC&pg=... https://archive.org/details/gravitation00misn_003 https://archive.org/details/gravitation00misn_003/... https://en.wikipedia.org/wiki/File:Perpendicular-c...