Triangle is a simple closed curve made up with 3 lines segment. it have 3 vertex, 3 side and 3 angle. In a triangle ABC, the side are AB, BC, CA. Angles are ∠BAC, ∠ABC, ∠BCA. Vertices are A,B,C. In the above triangle the side opposite to the vertex A is BC. please try to say the opposite side of other vertex.
Based on side triangle can classifieds in to Scalene ( all three sides are unequal), Isosceles (2 sides are same), And Equilateral triangle (3 side are same).
Based on Angles Triangle can classifieds in to Acute-angle, Obtuse-angle, and Right angle triangle. see page 113.
Medians of triangle
A median is the line segment between a vertex and the midpoint of the opposite side. A triangle can be 3 medians. If we cut a triangle in a sheet of paper we can fold each of its sides and can be find the midpoint of each side. see (P114) for think and discuss.
Altitude of a triangle
It is nothing but the height of the triangle. It is the line segment between the base and opposite vertex. See (P115) for think and discuss. See (P116) for exercise.