Revisiting Triangle : a Foundational Element of Plane Geometry

평면도형 탐구의 기본 요소로서 삼각형의 재조명

What is a foundational element of plane geometry? Isn't it possible to constitute the contents of plane geometry from that element? In this paper, we suggest a view point that triangle is a foundational element of plane geometry. And take some examples of reconstruction of usually given contents and mathematical activity centered on the triangle in plane geometry.