NCERT Class 9 Maths Formula CBSE Board Sample Problems Part 8
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8. Quadrilaterals
S.no  Terms  Descriptions 
1  Quadrilateral  A quadrilateral is the union of four linesegments determined by four distinct coplanar points of which no three are collinear and the linesegments intersect only at end points. For ABCD to be quadrilateral, following condition are required a) The four points A, B, C and D must be distinct and coplanar. b) No three of points A, B, C and D are colinear. c) Line segments i.e. AB, BC, CD, DA intersect at their end points only. A quadrilateral is a foursided polygon with four angles. There are many kinds of quadrilaterals. The five most common types are the parallelo gram, the rectangle, the square, the trapezoid, and the rhombus 
2  Angle Property of Quadrilateral  1) Sum of all the interior angles is 360° 2) Sum of all the exterior angles is 360° 
3  Parallelogram  A quadrilateral which has both pairs of opposite sides parallel is called a parallelogram. its properties are:
A quadrilateral is said to a parallelogram if Opposite sides are equal OR Opposite angles are equal OR Diagonal bisects each other OR A pair of opposite are parallel and equal 
4  Trapezium  A quadrilateral which has one pair of opposite sides parallel is called a trapezium. 
5  Rhombus  Rhombus is a parallelogram in which any pair of adjacent sides is equal. Properties of a rhombus:

6  Rectangles  A parallelogram which has one of its angles a right angle is called a rectangle. Properties of a rectangle are:

7  Square  A quadrilateral, all of whose sides are equal and all of whose angles are right angles. Properties of square are:
The diagonals of a square are equal. 
8  Important points about quadrilaterals  a) A square is always a parallelogram. b) A square is always a rectangle. c) A rhombus can be square. d) A rectangle has four right angles. 
9  Midpoint Theorems for Triangles  1)The line segment joining the midpoints of the two sides of the triangle is parallel to the third side 2) A line drawn through midpoint of one side of a triangle and parallel to another side bisect the third side of the triangle 