## Revision Notes ofÂ Chapter 8 Quadrilaterals Class 9th Math

**Topics in the Chapter**

- Quadrilaterals and its sides
- Types of Quadrilaterals
- Properties of Parallelogram
- Angle Sum Property of Quadrilateral
- Mid-Point Theorem for QuadrilateralÂ
- Theorems for Quadrilaterals

**Quadrilaterals**

In the given figure, we have:

(i) The points A, B, C and D are the vertices of quadrilateral ABCD.

(ii) The line segments AB, BC, CD and DA are the sides of quadrilateral ABCD.

(iii) The line segments AC and BD are called the diagonals of quadrilateral ABCD.

**Sides and Angles in Quadrilaterals:**

(i) Two sides having a common end point are called adjacent sides.

(ii) Two sides having no common end point are called opposite sides.

(iii) Two angles of a quadrilateral having a common arm are called consecutive angles.

(iv) Two angles of a quadrilateral having no common arm are called its opposite angles.

**Types of Quadrilaterals**

**(i) Parallelogram:**A quadrilateral in which opposite sides are parallel is called a parallelogram.

Here, AB || CD and AD || BC.

Also, opposite sides of a parallelogram are equal.

**(ii) Rectangle:**A parallelogram, each of whose angle is 90Âº, is called a rectangle.

**(iii) Square:**A rectangle having all sides equal is called a square.

**(iv) Rhombus:**A parallelogram having all sides equal is called a rhombus.

**(v) Trapezium:**A quadrilateral in which two opposite sides are parallel and two opposite sides are nonparallel, is called a trapezium.

__Note:__Â If the two non-parallel sides of a trapezium are equal, then it is called an isosceles trapezium.

**(vi) Kite:**A quadrilateral in which two pairs of adjacent sides are equal is known as kite.

**Facts to note:**

(i) A square, rectangle and rhombus are all parallelograms.

(ii) A square is a rectangle and also a rhombus, but a rectangle or a rhombus is not a square.

(iii) A parallelogram is a trapezium, but a trapezium is not a parallelogram.

(iv) A kite is not a parallelogram.

**Properties of Parallelogram**

(i) A diagonal of a parallelogram, divides it into two congruent triangles.

(ii) In a parallelogram, opposite sides are equal.

(iii) In a parallelogram, opposite angles are equal.

(iv) The diagonals of a parallelogram bisect each other.

**Angle Sum Property of Quadrilateral**

The sum of all the four angles of a quadrilateral is 360Âº

**Proof:**

Construction: Join BD.

Since, the sum of the angles of a triangle is 180Âº.

âˆ´ In Î”ABD, we have

âˆ 1 + âˆ A + âˆ 2 = 180ÂºÂ --- (1)

In Î”BCD, we have

âˆ 3 + âˆ C + âˆ 4 = 180ÂºÂ --- (2)

Adding (1) and (2), we get

[âˆ 1 + âˆ A + âˆ 2] + [âˆ 3 + âˆ C + âˆ 4] = 180Âº + 180Âº

(âˆ 1 + âˆ 3) + âˆ A + âˆ C + (âˆ 2 + âˆ 4) = 180Â° + 180Â°

â‡’ âˆ B + âˆ A + âˆ C + âˆ D = 360ÂºÂ

[âˆµ âˆ 1 + âˆ 3 = âˆ B and âˆ 2 + âˆ 4 = âˆ D]

âˆ´ âˆ A + âˆ B + âˆ C + âˆ D = 360Âº

Thus, the sum of four angles of a quadrilateral is 360Âº.

**Mid-Point Theorem for QuadrilateralÂ**

**Statement:**The line segment joining the mid-points of any two sides of a triangle is parallel to the third side and equal to half of it.

**Proof:Â**

**Construction:**Let us join DE and produce it. Draw CF || BA to meet DE (produced) in F.

We have to prove that DE = (1/2)BC and DE || BC.

Now, in Î”AED and Î”CEF, we have

AE = CEÂ Â [Given]

âˆ AED = âˆ CEFÂ Â [Vertically opposite angles]

âˆ DAE = âˆ FCEÂ Â [âˆµ AB || CF and AC is a transversal]

âˆ´ Î”AED â‰Œ Î”CEFÂ [ASA criteria]

â‡’ Their corresponding parts are equal.

â‡’ AD = CF and DE = EF

But AD = BDÂ Â [âˆµ D is mid point of AB]

â‡’ BD = CF Also, BD || CFÂ Â [By construction]

â‡’ BCFD is a parallelogram.

âˆ´ DF = BC and DF || BC

â‡’ (1/2)(DF) = (1/2)

(BC) and DE || BCÂ Â [âˆµ DE = EF]

â‡’ DE = (1/2)

BC and DE || BC Hence, the line segment joining the mid-points of any two sides of a triangle is parallel to the third side and equal to half of it.

**Converse of mid-point theorem:**The line drawn through the mid-point of one side of a triangle parallel to another side, intersects the third side at its mid-point.

**NCERT Solutions for Quadrilaterals Class 9**