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## Maths Class 9 Notes for Quadrilaterals

A quadrilateral is a closed figure obtained by joining four point (with no three points collinear) in an order.

Every quadrilateral has : (i) Four vertices, (ii) Four sides, (iii) Four angles and (iv) Two diagonals.

S**UM OF THE ANGLES OF A QUADRILATERAL**

**Statement:** The sum of the angles ofa quadrilateral is 360°

**TYPES OF QUADRILATERALS**

1. **Trapezium :** It is quadrilateral in which one pair of opposite sides are parallel.

2. **Parallelogram :** It is a quadrilateral in which both the pairs of opposite sides are parallel.

3. **Rectangle :** It is a quadrilateral whose each angle is 90°. ABCD is a rectangle.

(i) ∠A+ ∠B = 90° + 90° = 180° ⇔ AD || BC

(ii) ∠B+ ∠C= 900 + 900 = 180° ⇔ AB || DC

Rectangle ABCD is a parallelogram also.

4. **Rhombus :** It is a quadrilateral whose all the sides are equal.

5. **Square :** It is a quadrilateral whose all the sides are equal and each angle is 90°.

6. **Kite :** It is a quadrilateral in which two pairs of adjacent sides are equal.

**Note :**

- Square, rectangle and rhombus are all parallelograms.
- Kite and trapezium are not parallelograms.
- A square is a rectangle.
- A square is a rhombus.
- A parallelogram is a trapezium.

**PARALLELOGRAM:**

A parallelogram is a quadrilateral in which opposite sides are parallel. It is denoted by

**PROPERTIES OF PARALLELOGRAM:**

1. A diagonal of a parallelogram divides it into two congruent triangles.

2. The opposite sides of a parallelogram are equal.

**Theorem :** If each pair of opposite sides of a quadrilateral is equal, then it is a parallelogram.

3. The opposite angles of a parallelogram are equal.

**Theorem :** If in a quadrilateral, each pair of opposite angles is equal, then it is a parallelogram.

4. The diagonals of a parallelogram bisect each other.

**Theorem :** If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.

**MID POINT THEOREM**

**(BASIC PROPORTIONALITY THEOREM)**

**Statement 1:**

The line segment joining the mid-points of any two sides of a triangle is parallel to the third side

**Statement 2:**

The line drawn through the mid-point of one side of a triangle, parallel to another side bisects the third side.

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