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TRIANGLE, in geometry; a figure of three sides and three angles. Triangles are either plane or spherical. A plane triangle is one contained by three right lines; and a spherical triangle is one contained by three arcs of great cir cles of the sphere. Triangles are denominated, from their angles, right, obtuse, and acute. A rightangled triangle is that which hask one right angle ; an obtuseangled triangle is such as has one ob^ tuse angle ; and an acuteangled triangle is that which has all its angles acute. The triangle is the most important figurein geometry ; and its various lines bear the most interesting relations to each other. (See Trigonometry.)