1. The angle whose measurement is of is called RIGHT ANGLE.
2. The angle whose measurement is greater than is called OBTUSE ANGLE.
3. The angle whose measurement is less than is called ACUTE ANGLE.
4. If , and are the midpoints of the sides of , then , and are called Medians of the triangles and medians of a triangles are concurrent at the point
5. If , and are the perpendiculars from , , to the sides , and of respectively, then , and are called ALTITUDES OF THE TRIANGLE.
6. The line passing through the mid points of the sides of , and perpendicular to the respective sides are called RIGHT BISECTORS OF THE TRIANGLE.
7. Angle bisector of a triangle are concurrent at .
8. The triangle whose one angle is is called a RIGHT ANGLED TRIANGLE.
9. The triangle, whose sides are of equal length, is called EQUILATERAL TRIANGLE.
10. The triangle whose two sides or two angles are equal of measures, is called ISOSCELES TRIANGLE.
11. If , and are the internal angles of a triangle then