Ah, those terms from 3D geometry. Direction cosines are the cosines of angles a line makes with the axes, and direction ratios are numbers proportional to them. The sum of squares equals one because it comes from a trigonometric identity. Truthfully, I have forgotten the exact proof. In my ledger, a column must balance to one, and that is the only sum I worry over now. These formulas feel as distant as my own engineering dreams, set aside for my beta's books.
#exams#study
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