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