11.05 Classifying quadrilaterals

A quadrilateral is a four-sided 2D figure. By showing that quadrilaterals have specific properties, we can narrow it down to the most specific type of quadrilateral.

The hierarchy of parallelograms

We define a parallelogram as any quadrilateral whose opposite side lengths are parallel. In some instances, a parallelogram can also be a rhombus, rectangle or square, depending on what knowledge we know about the parallelogram.

Properties of parallelograms:

• Opposite sides are parallel
• Opposite sides are congruent
• Opposite angles are congruent
• Consecutive angles are supplementary
• The diagonals bisect each other

We define a rhombus as a parallelogram whose all four sides are congruent.

Rhombus

Properties of a rhombus:

• Four congruent sides
• Diagonals are perpendicular
• Each diagonal bisects a pair of opposite angles

We define a rectangle as a parallelogram whose interior angles are right angles.

Rectangle

Properties of rectangles:

• Four right angles
• Diagonals are congruent
• Diagonals bisect each other

And lastly, we define a square as a parallelogram whose all four sides are congruent and and all interior angles are right angles.

Square

Properties of squares:

• Four congruent sides and four congruent angles
• A special case of both a rectangle and a rhombus
• Diagonals are perpendicular

 

If we can prove that a parallelogram possesses one of these necessary conditions, then we can prove that it is a rhombus, rectangle or square respectively.

 

Exploration

Consider the following theorem.

If a parallelogram has one pair of consecutive sides that are congruent, then the parallelogram is a rhombus.

To begin proving the above theorem, we can first label the parallelogram by its set of vertices, $PQRS$PQRS.

Parallelogram $PQRS$PQRS

From the given information, we know that a pair of consecutive sides are congruent. So let's choose $\overline{PQ}$PQ and $\overline{SP}$SP. Ultimately we could have chosen another pair, but the proof would be the same. We write this as a geometric statement $\overline{PQ}\cong\overline{SP}$PQ≅SP.

Parallelogram $PQRS$PQRS with congruent sides

 

We also know that the $PQRS$PQRS is a parallelogram, and one feature of the parallelogram is that opposite sides are congruent. We can state this as $\overline{PQ}\cong\overline{RS}$PQ≅RS and $\overline{SP}\cong\overline{QR}$SP≅QR. Congruence is transitive, meaning that if two things are congruent to the same thing, they are also congruent. This means that $\overline{PQ}\cong\overline{QR}$PQ≅QR and $\overline{SP}\cong\overline{RS}$SP≅RS and more generally all the sides are congruent. By definition, a rhombus is a parallelogram with all sides congruent. So $PQRS$PQRS is a rhombus and we are finished with the proof.

We can formalize the above steps into a two-column proof where each line contains a geometric statement in the left column and a corresponding reason in the right column.

 

Two-column proof

Given the parallelogram $PQRS$PQRS, and the fact that $\overline{PQ}\cong\overline{SP}$PQ≅SP, prove that $PQRS$PQRS is a rhombus.
Statements	Reasons
$PQRS$PQRS is  a parallelogram and $\overline{PQ}\cong\overline{SP}$PQ≅SP	Given
$\overline{PQ}\cong\overline{RS}$PQ≅RS and $\overline{SP}\cong\overline{QR}$SP≅QR	If a quadrilateral is a parallelogram, then its opposite sides are congruent.
$\overline{PQ}\cong\overline{QR}\cong\overline{RS}\cong\overline{SP}$PQ≅QR≅RS≅SP	Transitive property of congruence
$PQRS$PQRS is a rhombus	Definition of a rhombus

The final line contains the statement that the quadrilateral is a rhombus, which is what we wanted to prove.

Practice questions

question 1

question 2

 

The hierarchy of trapezoids

We define a trapezoid as any quadrilateral with one pair of opposite sides that are parallel.

A special type of a trapezoid, called an isosceles trapezoid, occurs when the two legs of the trapezoid are congruent and hence the diagonals are congruent.

Isosceles trapezoid

 

There are a number of theorems that explain equivalent ways of identifying whether a trapezoid is isosceles. We call these special theorems criteria.

1. If a trapezoid has congruent base angles, then it is an isosceles trapezoid.

Trapezoid with base angles congruent

 

2. If a trapezoid has opposite angles that are supplementary, then it is an isosceles trapezoid.

Trapezoid with $\angle1$∠1 and $\angle3$∠3 are supplementary, and $\angle2$∠2 and $\angle4$∠4 are supplementary

 

3. If a trapezoid has congruent diagonals, then it is an isosceles trapezoid.

Trapezoid with congruent diagonals

 

Exploration

Consider the following theorem.

If a trapezoid has congruent base angles, then it is an isosceles trapezoid.

To begin proving the above theorem, we can first label the trapezoid by its set of vertices, $ABCD$ABCD. Let $\angle ADC$∠ADC and $\angle BCD$∠BCD be congruent.

Trapezoid $ABCD$ABCD

 

We next construct the pair of altitudes $\overline{AP}$AP and $\overline{BQ}$BQ, which form the pair of triangles $\triangle APD$△APD and $\triangle BQC$△BQC. If we can show that these triangles are congruent, then we can show that $\overline{AD}$AD is congruent to $\overline{BC}$BC, which will mean that $ABCD$ABCD is an isosceles trapezoid.

Trapezoid $ABCD$ABCD with altitudes drawn

 

Firstly we are given that $\angle ADC$∠ADC and $\angle BCD$∠BCD are congruent. Since altitudes create right angles with the base of a trapezoid, $\angle APD$∠APD and $\angle BQC$∠BQC are right angles. All right angles are congruent, so we have that $\angle APD$∠APD and $\angle BQC$∠BQC are congruent. The altitudes themselves are congruent, so $\overline{AP}$AP is congruent to $\overline{BQ}$BQ. This means that $\triangle APD\cong\triangle BQC$△APD≅△BQC are congruent by angle-angle-side congruence theorem.

It follows that the two sides $\overline{AD}$AD and $\overline{BC}$BC are congruent because they are corresponding parts of the two congruent triangles. Hence the trapezoid $ABCD$ABCD is isosceles, by definition.

 

We can formalize the above steps into a two-column proof where each line contains a geometric statement in the left column and a corresponding reason in the right column.

Statements	Reasons
$ABCD$ABCD is a trapezoid, $\overline{AP}$AP and $\overline{BQ}$BQ are altitudes, and $\angle ADC\cong\angle BCD$∠ADC≅∠BCD	Given
$\angle APD$∠APD and $\angle BQC$∠BQC are right angles	Definition of an altitude
$\angle APD\cong\angle BQC$∠APD≅∠BQC	All right angles are congruent.
$\overline{AP}\cong\overline{BQ}$AP≅BQ	Altitudes of a trapezoid are congruent
$\triangle APD\cong\triangle BQC$△APD≅△BQC	Angle-angle-side congruence theorem
$\overline{AD}\cong\overline{BC}$AD≅BC	Corresponding parts of congruent triangles are congruent (CPCTC)
$ABCD$ABCD is an isosceles trapezoid	Definition of an isosceles trapezoid

 

Below is a summary of the criteria for a trapezoid to be an isosceles trapezoid.

Practice Questions

Question 1

Question 2