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Millian, Richard S.; Parker, George D. (1981). Geometry: A Metric Approach with Models. Undergraduate Texts in Mathematics. New York: Springer. p. 133. doi:10.1007/978-1-4684-0130-1. ISBN978-1-4684-0130-1.
"Triangle (geometry)" . Encyclopædia Britannica. Vol. 27 (11th ed.). 1911. p. 258. ...Also a right-angled triangle has one angle a right angle, the side opposite this angle being called the hypotenuse;...