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In Euclidean geometry, the base angles can not be obtuse (greater than 90°) or right (equal to 90°) because their measures would sum to at least 180°, the total of all angles in any Euclidean triangle. [8] Since a triangle is obtuse or right if and only if one of its angles is obtuse or right, respectively, an isosceles triangle is obtuse ...
This is an SVG drawing of a scalene triangle with sides and angles marked, last of six-image series with Image:Triangle-acute.svg, Image:Triangle-obtuse.svg, Image:Triangle-right.svg, Image:Triangle-isosceles.svg, and Image:Triangle-equilateral.svg. All files are the same size, 505 by 440.
An obtuse triangle (or obtuse-angled triangle) is a triangle with one obtuse angle (greater than 90°) and two acute angles. Since a triangle's angles must sum to 180° in Euclidean geometry , no Euclidean triangle can have more than one obtuse angle.
A triangle in which one of the angles is a right angle is a right triangle, a triangle in which all of its angles are less than that angle is an acute triangle, and a triangle in which one of it angles is greater than that angle is an obtuse triangle. [8] These definitions date back at least to Euclid. [9]
English: Euler diagram of types of triangles, using the definition that isosceles triangles are triangles with at least 2 equal sides (that is, equilateral triangles are isosceles triangles). Date 17 April 2011
When obtuse triangles are glued in this way, the resulting surface can be folded to form a disphenoid (by Alexandrov's uniqueness theorem) but one with acute triangle faces and with edges that in general do not lie along the edges of the given obtuse triangles. Two more types of tetrahedron generalize the disphenoid and have similar names.
The heptagonal triangle's orthic triangle, with vertices at the feet of the altitudes, is similar to the heptagonal triangle, with similarity ratio 1:2. The heptagonal triangle is the only obtuse triangle that is similar to its orthic triangle (the equilateral triangle being the only acute one). [2]: pp. 12–13
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