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6.4 Rhombuses, Rectangles and Squares. Geometry CCSS: G.CO 11.

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6 4 rhombuses rectangles and squares

6.4 Rhombuses, Rectangles and Squares

Geometry CCSS: G.CO 11.

PROVE theorems about parallelograms. Theorems INCLUDE: opposite sides ARE congruent, opposite angles ARE congruent, the diagonals of a parallelogram BISECT each other, and conversely, rectangles ARE parallelograms with congruent diagonals.


U e q
U.E.Q

What are the properties of different quadrilaterals? How do we use the formulas of areas of different quadrilaterals to solve real-life problems?

Now a little review:


Standards for mathematical practice
Standards for Mathematical Practice

11 PROVE theorems about parallelograms. Theorems INCLUDE: opposite sides ARE congruent, opposite angles ARE congruent, the diagonals of a parallelogram BISECT each other, and conversely, rectangles ARE parallelograms with congruent diagonals.

  • 1. Make sense of problems and persevere in solving them.

  • 2.Reason abstractly and quantitatively.

  • 3. Construct viable arguments and critique the reasoning of others.

  • 4. Model with mathematics.

  • 5. Use appropriate tools strategically.

  • 6.Attend to precision.

  • 7. Look for and make use of structure.

  • 8.Look for and express regularity in repeated reasoning.


6

6

Do you have enough information?

Yes

Yes

No

Yes


50°

130°

Do you have enough information?

Yes

No

Yes

Yes


Activator
Activator

Test your prior knowledge and try to fill in the chart with properties of the following quadrilaterals:


Objectives
Objectives:

Use properties of sides and angles of rhombuses, rectangles, and squares.

Use properties of diagonals of rhombuses, rectangles and squares.


E.Q

What are properties of sides and angles of rhombuses, rectangles, and squares?


Properties of special parallelograms
Properties of Special Parallelograms

A rectangle is a parallelogram with four right angles.

A rhombus is a parallelogram

with four congruent sides

A square is a parallelogram with four congruent sides and four right angles.

In this lesson, you will study three special types of parallelograms: rhombuses, rectangles and squares.


Venn diagram shows relationships memorize
Venn Diagram shows relationships-- MEMORIZE

  • Each shape has the properties of every group that it belongs to. For instance, a square is a rectangle, a rhombus and a parallelogram; so it has all of the properties of those shapes.

parallelograms

rhombuses

rectangles

squares





Ex 1 describing a special parallelogram
Ex. 1: Describing a special parallelogram

  • Decide whether the statement is always, sometimes, or never true.

  • A rhombus is a rectangle.

  • A parallelogram is a rectangle.

parallelograms

rhombuses

rectangles

squares


Ex 1 describing a special parallelogram1
Ex. 1: Describing a special parallelogram

  • Decide whether the statement is always, sometimes, or never true.

  • A rhombus is a rectangle.

    The statement is sometimes true. In the Venn diagram, the regions for rhombuses and rectangles overlap. IF the rhombus is a square, it is a rectangle.

parallelograms

rhombuses

rectangles

squares


Ex 1 describing a special parallelogram2
Ex. 1: Describing a special parallelogram

  • Decide whether the statement is always, sometimes, or never true.

  • A parallelogram is a rectangle.

    The statement is sometimes true. Some parallelograms are rectangles. In the Venn diagram, you can see that some of the shapes in the parallelogram box are in the area for rectangles, but many aren’t.

parallelograms

rhombuses

rectangles

squares


Ex 2 using properties of special parallelograms
Ex. 2: Using properties of special parallelograms

  • ABCD is a rectangle. What else do you know about ABCD?

  • Because ABCD is a rectangle, it has four right angles by definition. The definition also states that rectangles are parallelograms, so ABCD has all the properties of a parallelogram:

    • Opposite sides are parallel and congruent.

    • Opposite angles are congruent and consecutive angles are supplementary.

    • Diagonals bisect each other.


Take note
Take note:

  • A rectangle is defined as a parallelogram with four right angles. But any quadrilateral with four right angles is a rectangle because any quadrilateral with four right angles is a parallelogram.

  • Corollaries about special quadrilaterals:

    • Rhombus Corollary: A quadrilateral is a rhombus if and only if it has four congruent sides.

    • Rectangle Corollary: A quadrilateral is a rectangle if and only if it has four right angles.

    • Square Corollary: A quadrilateral is a square if and only if it is a rhombus and a rectangle.

    • You can use these to prove that a quadrilateral is a rhombus, rectangle or square without proving first that the quadrilateral is a parallelogram.


Ex 3 using properties of a rhombus
Ex. 3: Using properties of a Rhombus

In the diagram at the right,

PQRS is a rhombus. What

is the value of y?

All four sides of a rhombus are ≅, so RS = PS.

5y – 6 = 2y + 3 Equate lengths of ≅ sides.

5y = 2y + 9 Add 6 to each side.

3y = 9 Subtract 2y from each side.

y = 3 Divide each side by 3.


Using diagonals of special parallelograms
Using diagonals of special parallelograms

  • The following theorems are about diagonals of rhombuses and rectangles.

  • Theorem 6.11: A parallelogram is a rhombus if and only if its diagonals are perpendicular.

  • ABCD is a rhombus if and only if AC BD.


Using diagonals of special parallelograms1
Using diagonals of special parallelograms

  • Theorem 6.12: A parallelogram is a rhombus if and only if each diagonal bisects a pair of opposite angles.

  • ABCD is a rhombus if and only if AC bisects DAB and BCD and BD bisects ADC and CBA.


Using diagonals of special parallelograms2
Using diagonals of special parallelograms

A

B

  • Theorem 6.13: A parallelogram is a rectangle if and only if its diagonals are congruent.

  • ABCD is a rectangle if and only if AC ≅ BD.

D

C


NOTE:

You can rewrite Theorem 6.11 as a conditional statement and its converse.

Conditional statement: If the diagonals of a parallelogram are perpendicular, then the parallelogram is a rhombus.

Converse: If a parallelogram is a rhombus, then its diagonals are perpendicular.


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd1
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd2
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given

  • Def. of . Diagonals bisect each other.


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd3
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given

  • Def. of . Diagonals bisect each other.

  • Def. of . Diagonals bisect each other.


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd4
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given

  • Def. of . Diagonals bisect each other.

  • Def. of . Diagonals bisect each other.

  • SSS congruence post.


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd5
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given

  • Def. of . Diagonals bisect each other.

  • Def. of . Diagonals bisect each other.

  • SSS congruence post.

  • CPCTC


Ex 4 proving theorem 6 11 given abcd is a rhombus prove ac bd6
Ex. 4: Proving Theorem 6.11Given: ABCD is a rhombusProve: AC  BD

Statements:

  • ABCD is a rhombus

  • AB ≅ CB

  • AX ≅ CX

  • BX ≅ DX

  • ∆AXB ≅ ∆CXB

  • AXB ≅ CXB

  • AC  BD

Reasons:

  • Given

  • Given

  • Def. of . Diagonals bisect each other.

  • Def. of . Diagonals bisect each other.

  • SSS congruence post.

  • CPCTC

  • Congruent Adjacent s


Ex 5 coordinate proof of theorem 6 11 given abcd is a parallelogram ac bd prove abcd is a rhombus
Ex. 5: Coordinate Proof of Theorem 6.11Given: ABCD is a parallelogram, AC  BD.Prove: ABCD is a rhombus

  • Assign coordinates. Because AC BD, place ABCD in the coordinate plane so AC and BD lie on the axes and their intersection is at the origin.

  • Let (0, a) be the coordinates of A, and let (b, 0) be the coordinates of B.

  • Because ABCD is a parallelogram, the diagonals bisect each other and OA = OC. So, the coordinates of C are (0, - a). Similarly the coordinates of D are (- b, 0).

A(0, a)

D(- b, 0)

B(b, 0)

C(0, - a)


Ex 5 coordinate proof of theorem 6 11 given abcd is a parallelogram ac bd prove abcd is a rhombus1
Ex. 5: Coordinate Proof of Theorem 6.11Given: ABCD is a parallelogram, AC  BD.Prove: ABCD is a rhombus

  • Find the lengths of the sides of ABCD. Use the distance formula (See – you’re never going to get rid of this)

    AB=√(b – 0)2 + (0 – a)2 = √b2 + a2

  • BC= √(0 - b)2 + (– a - 0)2 = √b2 + a2

  • CD= √(- b – 0)2 + [0 - (– a)]2 = √b2 + a2

  • DA= √[(0 – (- b)]2 + (a – 0)2 = √b2 + a2

A(0, a)

D(- b, 0)

B(b, 0)

C(0, - a)

All the side lengths are equal, so ABCD is a rhombus.


Ex 6 checking a rectangle
Ex 6: Checking a rectangle

4 feet

  • CARPENTRY. You are building a rectangular frame for a theater set.

  • First, you nail four pieces of wood together as shown at the right. What is the shape of the frame?

  • To make sure the frame is a rectangle, you measure the diagonals. One is 7 feet 4 inches. The other is 7 feet 2 inches. Is the frame a rectangle? Explain.

6 feet

6 feet

4 feet


Ex 6 checking a rectangle1
Ex 6: Checking a rectangle

4 feet

  • First, you nail four pieces of wood together as shown at the right. What is the shape of the frame?

    Opposite sides are congruent, so the frame is a parallelogram.

6 feet

6 feet

4 feet


Ex 6 checking a rectangle2
Ex 6: Checking a rectangle

4 feet

  • To make sure the frame is a rectangle, you measure the diagonals. One is 7 feet 4 inches. The other is 7 feet 2 inches. Is the frame a rectangle? Explain.

    The parallelogram is NOT a rectangle. If it were a rectangle, the diagonals would be congruent.

6 feet

6 feet

4 feet


You’ve just had a new door installed, but it doesn’t seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?


Foldable
Foldable seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

1. Take out a piece of notebook paper and make a hot dog fold over from the right side over to the pink line.


Foldable1

The fold crease seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

2. Now, divide the right hand section into 5 sections by drawing 4 evenly spaced lines.

3. Use scissors to cut along your drawn line, but ONLY to the crease!


Foldable2

The fold crease seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

4. Write QUADRILATERALS down the left hand side

QUADRILATERALS


Foldable3

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

The fold crease

Foldable

5. Fold over the top cut section and write PARALLELOGRAM on the outside.

Parallelogram

6. Reopen the fold.


Foldable4

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

7. On the left hand section, draw a parallelogram.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

8. On the right hand side, list all of the properties of a parallelogram.


Foldable5

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* Fold over the second cut section and write RECTANGLE on the outside.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

RECTANGLE

* Reopen the fold.


Foldable6

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* On the left hand section, draw a rectangle.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

1. Special parallelogram.

2. Has 4 right angles

3. Diagonals are congruent.

* On the right hand side, list all of the properties of a rectangle.


Foldable7

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* Fold over the third cut section and write RHOMBUS on the outside.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

1. Special parallelogram.

2. Has 4 right angles

3. Diagonals are congruent.

* Reopen the fold.

RHOMBUS


Foldable8

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* On the left hand section, draw a rhombus.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

1. Special parallelogram.

2. Has 4 right angles

3. Diagonals are congruent.

* On the right hand side, list all of the properties of a rhombus.

1. Special Parallelogram

2. Has 4 Congruent sides

3. Diagonals are perpendicular.

4. Diagonals bisect opposite angles


Foldable9

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* Fold over the third cut section and write SQUARE on the outside.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

1. Special parallelogram.

2. Has 4 right angles

3. Diagonals are congruent.

* Reopen the fold.

1. Special Parallelogram

2. Has 4 Congruent sides

3. Diagonals are perpendicular.

4. Diagonals bisect opposite angles

SQUARE


Foldable10

QUADRILATERALS seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Foldable

* On the left hand section, draw a square.

1. Opposite angles are congruent.

2. Consecutive angles are supplementary.

3. Opposite sides are congruent.

4. Diagonals bisect each other.

5. Opposite sides are parallel

1. Special parallelogram.

2. Has 4 right angles

3. Diagonals are congruent.

* On the right hand side, list all of the properties of a square.

1. Special Parallelogram

2. Has 4 Congruent sides

3. Diagonals are perpendicular.

4. Diagonals bisect opposite angles

* Place in your notebook and save for tomorrow.

1. All the properties of parallelogram, rectangle, and rhombus

2. 4 congruent sides and 4 right angles


Warm up

Warm-Up seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

Name the figure described.

A quadrilateral that is both a rhombus and a rectangle.

A quadrilateral with exactly one pair of parallel sides.

A parallelogram with perpendicular diagonals


6 5 trapezoids and kites

6.5 Trapezoids and Kites seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?


U e q1
U.E.Q seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

What are the properties of different quadrilaterals? How do we use the formulas of areas of different quadrilaterals to solve real-life problems?


E.Q seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

What are some properties of trapezoids and kits?


Objectives1
Objectives: seem to fit into the door jamb properly. What could you do to determine if your new door is rectangular?

  • Use properties of trapezoids.

  • Use properties of kites.


Using properties of trapezoids

A trapezoid is a quadrilateral with exactly one pair of parallel sides. The parallel sides are the bases. A trapezoid has two pairs of base angles. For instance in trapezoid ABCD D and C are one pair of base angles. The other pair is A and B. The nonparallel sides are the legs of the trapezoid.

Using properties of trapezoids


Using properties of trapezoids1

If the legs of a trapezoid are congruent, then the trapezoid is an isosceles trapezoid.

Using properties of trapezoids


Trapezoid theorems

Theorem 6.14 is an isosceles trapezoid.

If a trapezoid is isosceles, then each pair of base angles is congruent.

A ≅ B, C ≅ D

Trapezoid Theorems


Trapezoid theorems1

Theorem 6.15 is an isosceles trapezoid.

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

ABCD is an isosceles trapezoid

Trapezoid Theorems


Trapezoid theorems2

Theorem 6.16 is an isosceles trapezoid.

A trapezoid is isosceles if and only if its diagonals are congruent.

ABCD is isosceles if and only if AC ≅ BD.

Trapezoid Theorems


Midsegment of a trapezoid

The midsegment of a trapezoid is the segment that connects the midpoints of its legs. Theorem 6.17 is similar to the Midsegment Theorem for triangles.

Midsegment of a trapezoid


Theorem 6 17 midsegment of a trapezoid

The midsegment of a trapezoid is parallel to each base and its length is one half the sums of the lengths of the bases.

MN║AD, MN║BC

MN = ½ (AD + BC)

Theorem 6.17: Midsegment of a trapezoid


Ex 3 finding midsegment lengths of trapezoids

LAYER CAKE its length is one half the sums of the lengths of the bases. A baker is making a cake like the one at the right. The top layer has a diameter of 8 inches and the bottom layer has a diameter of 20 inches. How big should the middle layer be?

Ex. 3: Finding Midsegment lengths of trapezoids


Ex 3 finding midsegment lengths of trapezoids1

Use the midsegment theorem for trapezoids. its length is one half the sums of the lengths of the bases.

DG = ½(EF + CH)=

½ (8 + 20) = 14”

Ex. 3: Finding Midsegment lengths of trapezoids

E

F

D

G

D

C


Using properties of kites

A kite is a quadrilateral that has two pairs of consecutive congruent sides, but opposite sides are not congruent.

Using properties of kites


Kite theorems

Theorem 6.18 congruent sides, but opposite sides are not congruent.

If a quadrilateral is a kite, then its diagonals are perpendicular.

AC  BD

Kite theorems


Kite theorems1

Theorem 6.19 congruent sides, but opposite sides are not congruent.

If a quadrilateral is a kite, then exactly one pair of opposite angles is congruent.

A ≅ C, B ≅ D

Kite theorems


Ex 4 using the diagonals of a kite

WXYZ is a kite so the diagonals are perpendicular. You can use the Pythagorean Theorem to find the side lengths.

WX = √202 + 122≈ 23.32

XY = √122 + 122≈ 16.97

Because WXYZ is a kite, WZ = WX ≈ 23.32, and ZY = XY ≈ 16.97

Ex. 4: Using the diagonals of a kite


Ex 5 angles of a kite
Ex. 5: Angles of a kite use the Pythagorean Theorem to find the side lengths.

  • Find mG and mJ

    in the diagram at the

    right.

    SOLUTION:

    GHJK is a kite, so G ≅ J and mG = mJ.

    2(mG) + 132° + 60° = 360°Sum of measures of int. s of a quad. is 360°

    2(mG) = 168°Simplify

    mG = 84° Divide each side by 2.

    So, mJ = mG = 84°

132°

60°


6 6 special quadrilaterals

6.6 Special Quadrilaterals use the Pythagorean Theorem to find the side lengths.

Geometry


Standards for mathematical practice1
Standards for Mathematical Practice use the Pythagorean Theorem to find the side lengths.

11 PROVE theorems about parallelograms. Theorems INCLUDE: opposite sides ARE congruent, opposite angles ARE congruent, the diagonals of a parallelogram BISECT each other, and conversely, rectangles ARE parallelograms with congruent diagonals.

  • 1. Make sense of problems and persevere in solving them.

  • 2.Reason abstractly and quantitatively.

  • 3. Construct viable arguments and critique the reasoning of others.

  • 4. Model with mathematics.

  • 5. Use appropriate tools strategically.

  • 6.Attend to precision.

  • 7. Look for and make use of structure.

  • 8.Look for and express regularity in repeated reasoning.


Ccss g co 11

CCSS: G.CO.11 use the Pythagorean Theorem to find the side lengths.

PROVE theorems about parallelograms. Theorems INCLUDE: opposite sides ARE congruent, opposite angles ARE congruent, the diagonals of a parallelogram BISECT each other, and conversely, rectangles ARE parallelograms with congruent diagonals.


Warm up1
Warm-up use the Pythagorean Theorem to find the side lengths.

Name the figure:

1. a quadrilateral with exactly one pair of opposite angles congruent and perpendicular diagonals

2. a quadrilateral that is both a rhombus and a rectangle

3. a quadrilateral with exactly one pair of ll sides.

4. any ll’ogram with perpendicular diagonals.


Objectives2
Objectives: use the Pythagorean Theorem to find the side lengths.

  • Identify special quadrilaterals based on limited information.

  • Prove that a quadrilateral is a special type of quadrilateral, such as a rhombus or trapezoid.


E.Q: use the Pythagorean Theorem to find the side lengths.

How do we simplify real life tasks, checking if an object is rectangular, rhombus, trapezoid, kite or other quadrilateral?


Summarizing properties of quadrilaterals

In this chapter, you have studied the seven special types of quadrilaterals shown at the right. Notice that each shape has all the properties of the shapes linked above it. For instance, squares have the properties of rhombuses, rectangles, parallelograms, and quadrilaterals.

Summarizing Properties of Quadrilaterals

Quadrilateral

Trapezoid

Kite

Parallelogram

Rhombus

Rectangle

Isosceles trapezoid

Square


Ex 1 identifying quadrilaterals
Ex. 1: Identifying Quadrilaterals quadrilaterals shown at the right. Notice that each shape has all the properties of the shapes linked above it. For instance, squares have the properties of rhombuses, rectangles, parallelograms, and quadrilaterals.

  • Quadrilateral ABCD has at least one pair of opposite sides congruent. What kinds of quadrilaterals meet this condition?

Parallelogram

Rhombus

Opposites sides are ≅.

All sides are congruent.

Opposite sides are congruent.

Legs are congruent.

All sides are congruent.


Ex 2 connecting midpoints of sides

When you join the midpoints of the sides of any quadrilateral, what special quadrilateral is formed? Why?

Ex. 2: Connecting midpoints of sides


Ex 2 connecting midpoints of sides1

Solution: Let E, F, G, and H be the midpoints of the sides of any quadrilateral, ABCD as shown.

If you draw AC, the Midsegment Theorem for triangles says that FG║AC and EG║AC, so FG║EH. Similar reasoning shows that EF║HG.

So by definition, EFGH is a parallelogram.

Ex. 2: Connecting midpoints of sides


Proof with special quadrilaterals
Proof with Special Quadrilaterals of any quadrilateral, ABCD as shown.

  • When you want to prove that a quadrilateral has a specific shape, you can use either the definition of the shape as in example 2 or you can use a theorem.


Proving quadrilaterals are rhombuses
Proving Quadrilaterals are Rhombuses of any quadrilateral, ABCD as shown.

You have learned 3 ways to prove that a quadrilateral is a rhombus.

  • You can use the definition and show that the quadrilateral is a parallelogram that has four congruent sides. It is easier, however, to use the Rhombus Corollary and simply show that all four sides of the quadrilateral are congruent.

  • Show that the quadrilateral is a parallelogram and that the diagonals are perpendicular (Thm. 6.11)

  • Show that the quadrilateral is a parallelogram and that each diagonal bisects a pair of opposite angles. (Thm 6.12)


Ex 3 proving a quadrilateral is a rhombus

Show KLMN is a rhombus of any quadrilateral, ABCD as shown.

Solution: You can use any of the three ways described in the concept summary above. For instance, you could show that opposite sides have the same slope and that the diagonals are perpendicular. Another way shown in the next slide is to prove that all four sides have the same length.

AHA – DISTANCE FORMULA If you want, look on pg. 365 for the whole explanation of the distance formula

So, because LM=NK=MN=KL, KLMN is a rhombus.

Ex. 3: Proving a quadrilateral is a rhombus


Ex 4 identifying a quadrilateral

What type of quadrilateral is ABCD? Explain your reasoning. of any quadrilateral, ABCD as shown.

Ex. 4: Identifying a quadrilateral

60°

120°

120°

60°


Ex 4 identifying a quadrilateral1

What type of quadrilateral is ABCD? Explain your reasoning. of any quadrilateral, ABCD as shown.

Solution: A and D are supplementary, but A and B are not. So, AB║DC, but AD is not parallel to BC. By definition, ABCD is a trapezoid. Because base angles are congruent, ABCD is an isosceles trapezoid

Ex. 4: Identifying a quadrilateral

60°

120°

120°

60°


Ex 5 identifying a quadrilateral
Ex. 5: Identifying a Quadrilateral of any quadrilateral, ABCD as shown.

  • The diagonals of quadrilateral ABCD intersect at point N to produce four congruent segments: AN ≅ BN ≅ CN ≅ DN. What type of quadrilateral is ABCD? Prove that your answer is correct.

  • First Step: Draw a diagram. Draw the diagonals as described. Then connect the endpoints to draw quadrilateral ABCD.


Ex 5 identifying a quadrilateral1

First Step: Draw a diagram. Draw the diagonals as described. Then connect the endpoints to draw quadrilateral ABCD.

2nd Step: Make a conjecture:

Quadrilateral ABCD looks like a rectangle.

3rd step: Prove your conjecture

Given: AN ≅ BN ≅ CN ≅ DN

Prove ABCD is a rectangle.

Ex. 5: Identifying a Quadrilateral

B

C

N

A

D


Given an bn cn dn prove abcd is a rectangle

Because you are given information about diagonals, show that ABCD is a parallelogram with congruent diagonals.

First prove that ABCD is a parallelogram.

Because BN ≅ DN and AN ≅ CN, BD and AC bisect each other. Because the diagonals of ABCD bisect each other, ABCD is a parallelogram.

Then prove that the diagonals of ABCD are congruent.

From the given you can write BN = AN and DN = CN so, by the addition property of Equality, BN + DN = AN + CN. By the Segment Addition Postulate, BD = BN + DN and AC = AN + CN so, by substitution, BD = AC.

So, BD ≅ AC.

ABCD is a parallelogram with congruent diagonals, so ABCD is a rectangle.

Given: AN ≅ BN ≅ CN ≅ DNProve: ABCD is a rectangle.


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