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a^2 | angle = 60°}} In geometry, an equilateral triangle is a triangle in which all three sides are equal. In the familiar Euclidean geometry, equilateral triangles are also equiangular; that is, all three internal angles are also congruent to each other and are each 60°. They are regular polygons, and can therefore also be referred to as regular triangles. ==Principal properties== Denoting the common length of the sides of the equilateral triangle as ''a'', we can determine using the Pythagorean theorem that: * The area is * The perimeter is * The radius of the circumscribed circle is or * The geometric center of the triangle is the center of the circumscribed and inscribed circles * And the altitude (height) from any side is . Many of these quantities have simple relationships to the altitude ("h") of each vertex from the opposite side: * The area is * The radius of the circle circumscribing the three vertices is * The radius of the inscribed circle is In an equilateral triangle, the altitudes, the angle bisectors, the perpendicular bisectors and the medians to each side coincide. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「equilateral triangle」の詳細全文を読む スポンサード リンク
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