- Sum of the angles of a triangle is 180 degrees.
- Sum of any two sides of a triangle is greater than the third side.
- Pythagoras theorem:

In a right angle triangle,

(Hypotenuse)^2 = (base)^2 + (Height)^2 - The line joining the midpoint of a side of a triangle to the opposite vertex is called the MEDIAN
- The point where the three medians of a triangle meet is called CENTROID.

Centroid divides each of the medians in the ratio 2:1. - In an isosceles triangle, the altitude from the vertex bi-sects the base
- The median of a triangle divides it into two triangles of the same area.
- Area of a triangle formed by joining the midpoints of the sides of a given triangle is one-fourth of the area of the given triangle.

- The diagonals of a parallelogram bisects each other .
- Each diagonal of a parallelogram divides it into two triangles of the same area
- The diagonals of a rectangle are equal and bisect each other.
- The diagonals of a square are equal and bisect each other at right angles.
- The diagonals of a rhombus are unequal and bisect each other at right angles.
- A parallelogram and a rectangle on the same base and between the same parallels are equal in area.
- Of all the parallelograms of a given sides , the parallelogram which is a rectangle has the greatest area.