正せい三さん角かく錐すい

noun

regular triangular pyramid; triangular pyramid with equilateral base

A geometric solid: a pyramid whose base is an equilateral triangle and whose other faces are also equilateral triangles (a regular tetrahedron). Used in mathematics and geometry.

• 正せい三さん角かく錐すいの体たい積せきを求もとめる公こう式しきを覚おぼえていますか。

  Do you remember the formula for finding the volume of a regular triangular pyramid?

• この立りっ体たいは正せい三さん角かく錐すいで、すべての面めんが正せい三さん角かく形けいです。

  This solid is a regular triangular pyramid, and all its faces are equilateral triangles.