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The word "triangle" is so often taught as early as preschool that it is, for most children, a familiar term by the time they reach elementary school. But the "meaning" learned in preschool tends to be associated so strongly with particular images like these (left to right, the most to least familiar) that there is still new learning for the children to do. The tend not to recognize triangles that are extreme, or in unfamiliar positions, and they might well include as "triangles" figures that resemble their image even though they fail to fit the definition. They need also to see, explicitly the following:

Each of these is a triangle: Image:Triangles.png Image:TriangleUpsideDown.pngImage:Extreme_triangle.png

None of these is a triangle:   Image:Not-triangles1a.png

Formal definition:

A triangle is a polygon that has three sides. The restriction that it be a polygon is enough to exclude the cases (above) that are not triangles, but a definition alone is too terse for young children, not sufficient for them to build the right concept. Examples and non-examples, like the ones shown above, are necessary before the definition will have sufficient meaning.


Triangle Classifications

  Scalene Isosceles Equilateral
Acute Yes Yes Yes
Right Yes Yes No
Obtuse Yes Yes No

Initially, this leads to nine possible classes. Students often expect these classes to be mutually exclusive and non-empty. However, both ideas are incorrect. First, some combinations are not possible. For example, in an equilateral triangle, each angle measures 60 degrees. Hence, every equilateral triangle is an acute triangle and their are no examples of equilateral right triangles or obtuse equilateral triangles.

Second, some classes include others. For example, since every equilateral triangle has three congruent sides, it also has at least two congruent sides. Hence, every equilateral triangle is also an isosceles triangle. In terms of the classification system, the set of (acute) equilateral triangles is a subset of the set of acute isosceles triangles.

Mathematical background

An important theorem in Euclidean geometry is the Triangle Sum Theorem, which is a special case of the Polygon Sum Theorem.

Triangle Sum Theorem: The sum of the measures of the angles of a triangle is 180 degrees.

Students often discover this theorem by measuring the angles of several triangles using a protractor, or by using geometry software. Students can informally suggest this theorem by tearing off two angles of a triangle and aligning them with the third angle of the triangle to form a straight line.

In more advanced classes, students learn a proof of the Triangle Sum Theorem that involves drawing an auxiliary line and using properties of parallel lines to identify congruent angles. This reveals a surprising connection between parallel lines and triangles.

Other theorems about triangles include:

  • Exterior Angle Theorem
  • Third Angle Theorem

Related mathematical terms

Similar Triangles
Congruent Triangles