1.
equilateral triangle
A triangle in which all three sides are of equal length and, equivalently, all three interior angles are equal at 60°. The most symmetric of the triangle types and a foundational figure in elementary and middle-school geometry.
With a compass and a ruler, you can easily construct an equilateral triangle.
Composed of 正 (regular; correct, used as a geometric prefix meaning 'regular' as in 正方形 'square'), 三角 (triangle), and 形 (shape). The 正 prefix indicates that all sides and angles are equal, paralleling 'regular polygon' in English.
Usage
A core term in elementary and middle-school mathematics. The reading せいさんかくけい is the standard textbook reading, but せいさんかっけい (with sokuon) is also widely heard and is often listed as an alternative reading. Often appears alongside 直角三角形 (right triangle) and 二等辺三角形 (isosceles triangle) when classifying triangles. The 正〜形 pattern extends to 正方形 (regular square) and 正多角形 (regular polygon).
Common collocations
- 正三角形を描く: to draw an equilateral triangle
- 正三角形を作図する: to construct an equilateral triangle (in geometry)
- 正三角形の面積: area of an equilateral triangle
- 正三角形の一辺: one side of an equilateral triangle
Similar words
Related Words
Compare:
正方形 (square)