TRIANGLES. DEFINITIONS. 71. A Triangle is a plane figure bounded by three straight lines; as ABC. The sides of a triangle are the bounding lines. The Angles of a triangle are the angles formed by the sides meeting one another. A triangle has six parts,-three sides and three A angles. C B The Base of a triangle is the side upon which it is supposed to stand. The Vertical Angle of a triangle is the angle opposite the base. An Exterior Angle of a triangle is an angle formed by a side and an adjacent side produced; as a. The Vertex of a triangle is the angular point at the vertical angle. The Altitude of a triangle is the perpendicular distance from the vertex to the base, or to the base produced. Thus, CD is the altitude of both the triangles ABC and EBC. A Medial Line of a triangle is a line drawn from a vertex to the middle of the opposite side. 72. Triangles are classified as to their sides and angles. A Scalene Triangle is one which has no two sides equal. An Isosceles Triangle is one which has two sides equal. An Equilateral Triangle is one which has all its sides equal. An Acute-Angled Triangle is one which has three acute angles. Right. Obtuse. Acute. Equilateral Equiangular. A Right-Angled Triangle is one which has one right angle. An Obtuse-Angled Triangle is one which has one obtuse angle. An Equiangular Triangle is one which has all its angles equal. RELATION BETWEEN THE SIDES OF A TRIANGLE. THEOREM XIX. 73. Any side of a triangle is greater than the difference between the other two sides. |