The article below draws our attention on the problem that most of the I-st year students
don’t know the theorem of three perpendiculars and its reciprocals. We consider that the deficient
assimilation of this very theorem has as a main reason its lame methodological exposition in the school
manuals of mathematics. Here are formulated 7 reciprocal forms of the theorem of those three
perpendiculars establishing which of them is a theorem.
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