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.
正三角形を描く。
Draw an equilateral triangle.
正三角形の内角はすべて60度だ。
All interior angles of an equilateral triangle are 60 degrees.
コンパスと定規があれば正三角形は簡単に作図できる。
With a compass and a ruler, you can easily construct an equilateral triangle.
一辺の長さが分かれば、正三角形の面積や高さは公式を使って求められる。
If you know the length of one side, you can find the area and height of an equilateral triangle using a formula.
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:
- 三角形: triangle — the general term
- 二等辺三角形: isosceles triangle — two sides equal, the next-most-symmetric type
- 直角三角形: right triangle — has one 90° angle
- 正方形: square (regular quadrilateral) — built from the same 正〜形 pattern
- 正多角形: regular polygon — the broader category to which 正三角形 belongs