## Key Concepts

- Define a parallelogram.
- Find the relation between measures of sides of a parallelogram.
- Find the relation between the angles of a parallelogram.
- Explain other properties of parallelogram.

### Parallelogram

A quadrilateral whose two pairs of opposite sides are parallel is called a parallelogram.

#### Theorem

If a quadrilateral is a Properties of parallelograms, then its opposite sides are congruent.

**Proof: **In △DCA and △BAC,

∠1=∠3

AC=AC [Reflexive property]

∠2=∠4

Then,

△DCA≅ △BAC by Angle-Side-Angle congruence criterion.

If two triangles are congruent, their corresponding sides are equal.

So, AD− = BC−and AB− = CD−.

#### Theorem

If a quadrilateral is a parallelogram, then its consecutive angles are supplementary.

If

AB− ∥ CD−, AD− ∥ BC−

then

m∠A+m∠B=180°

m∠B+m∠C=180°

m∠C+m∠D=180°

m∠D+m∠A=180°

**Proof: **The angles ∠A and ∠B are the consecutive interior angles formed when parallel lines

AB− and CD− are intersected by transversal AD−.

We know that when two parallel lines are intersected by a transversal, the consecutive interior angles formed are supplementary.

So, ∠A+∠B=180°

#### Theorem

If a quadrilateral is a parallelogram, then opposite angles are congruent.

**Proof:** The angles ∠1 and ∠2 are the consecutive interior angles formed when parallel lines

PQ− and SR− are intersected by transversal PS−.

We know that when two parallel lines are intersected by a transversal, the consecutive interior angles formed are supplementary.

So, ∠1+∠2=180° …(1)

The angles ∠2 and ∠3 are the consecutive interior angles formed when parallel lines PS− and QR− are intersected by the transversal SR−

Therefore, ∠2 + ∠3 = 180° …(2)

From (1) and (2), we get,

∠1 + ∠2 = ∠2 + ∠3

∠1 = ∠3

So, opposite angles are equal.

#### Theorem

If a quadrilateral is a parallelogram, then its diagonals bisect each other.

**Proof: **In △PXS and △RXQ

∠1=∠3 [Consecutive interior angles formed when PS- and QR−are intersected by PR-]

PS=QR [If a quadrilateral is a parallelogram, then its opposite sides are congruent]

∠2=∠4 [Consecutive interior angles formed when PS- and QR− are intersected by QS−]

So, △PXS≅ △RXQ by Angle-Side-Angle criterion.

We know that if two triangles are congruent, their corresponding sides are equal.

Since △PXS≅ △RXQ then PX=XR and QX=XS

## Exercise

- Find the length of BC.

- In parallelogram WXYZ, find the measure of

- Find the measure of

- Find the perimeter of the parallelogram.

- What is the angle measure of the point at the bottom?

### Concept Map

### What we have learned

- A quadrilateral whose two pairs of opposite sides are parallel is called a parallelogram.
- If a quadrilateral is a parallelogram, then its opposite sides are congruent.
- If a quadrilateral is a parallelogram, then its consecutive angles are supplementary.
- If a quadrilateral is a parallelogram, then opposite angles are congruent.
- If a quadrilateral is a parallelogram, then its diagonals bisect each other.

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