Subject: Geometry

August 07, 2011 by matthew_suan -
(:cellnr bgcolor=#d4d7ba colspan=14 align=center:) '+'''The Breakdown'''+'
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*[[#t2 | Sum of all angles in a quadrilateral]]
*[[#t3 | Principles related to quadrilaterals]]
**[[#t4 | Principle 1]]
**[[#t5 | Principle 2]]
**[[#t6 | Principle 3]]
*[[#t7 | What are trapezoids?]]
*[[#t8 | Principles related to trapezoids]]
**[[#t9 | Principle 1]]
**[[#t10 | Principle 2]]
*[[#t11 | Applying Algebra to Trapezoid Figures]]
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In the following diagram, use your knowledge of geometry to find values for all four angles of each trapezoid by first finding values for {$x$} and {$y$}.
to:
In the following diagram, use your knowledge of geometry to find values for all four angles of each trapezoid by first finding values for {$x$} and {$y$}. These shouldn’t be too hard for you to solve, just use a little logic and you’ll be fine.
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PIC
FIG 9
Question 2.
to:

!!!Example
#2
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These shouldn’t be too hard for you to solve, just use a little logic and you’ll be fine.

Solutions are below!

Question 2.
to:
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Question 1.

The
statement that ABCD is isosceles has been given to us.
From this we can deduce two truths:
to:

The
statement that ABCD is isosceles has been given to us. From this we can deduce two truths:
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In the above diagram {$\angle{a} proportional to \angle{b}$}.
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In the above diagram {$\angle{a} \propto \angle{b}$}.
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Thus, in the diagram used above, if , then .
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Thus, in the diagram used above, if {$\angle{A} \propto \angle{B}$}, then {$\angle{A} \propto \angle{B}$} .
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PIC FIG 8
Question 1
.
to:

!!!Example #1
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PIC
FIG 9
Question
2.
As
before, work out the angle values for each corner.

These shouldn’t be too hard for you to solve, just use a little logic and you’ll be fine.

Solutions are below!

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1. also = 5x + 60°.
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1. {$\angle{C}= 5x + 60°$}.
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PIC – FIG 9
Question 2.
As before, work out the angle values for each corner.

These shouldn’t be too hard for you to solve, just use a little logic and you’ll be fine.

Solutions are below!

August 07, 2011 by matthew_suan -
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In the above diagram {$\angle{a} = \angle{b}$}.
to:
In the above diagram {$\angle{a} proportional to \angle{b}$}.
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Thus, in the diagram used above, if , then .'''
to:

Thus, in the diagram used above, if , then .
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%cframe width=585px%'''The base angles of an isosceles trapezoid are congruent.
In the above diagram
.'''
to:
%cframe width=585px%'''The base angles of an isosceles trapezoid are congruent.'''
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In the above diagram {$\angle{a} = \angle{b}$}.
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%cframe width=585px%'''Any trapezoid with congruent base angles is an isosceles trapezoid.
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%cframe width=585px%'''Any trapezoid with congruent base angles is an isosceles trapezoid.'''
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In the following diagram, use your knowledge of geometry to find values for all four angles of each trapezoid by first finding values for x and y.
to:
In the following diagram, use your knowledge of geometry to find values for all four angles of each trapezoid by first finding values for {$x$} and {$y$}.
August 07, 2011 by matthew_suan -
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%cframe width=585px% '''The base angles of an isosceles trapezoid are congruent.
to:
%cframe width=585px%'''The base angles of an isosceles trapezoid are congruent.
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%cframe width=585px% '''Any trapezoid with congruent base angles is an isosceles trapezoid.
to:
%cframe width=585px%'''Any trapezoid with congruent base angles is an isosceles trapezoid.
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The base angles of a trapezoid are the angles at the ends of its longer base, in Trapezoid Y’s case, {$\angle{C}$} and {$angle{D}$} .
to:
The base angles of a trapezoid are the angles at the ends of its longer base, in Trapezoid Y’s case, {$\angle{C}$} and {$\angle{D}$} .
August 07, 2011 by matthew_suan -
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!!Trapezoids
to:
!!What are trapezoids?
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In the following diagram showing Trapezoid Y, and are the legs, and are the bases, and is the median.

PIC FIG 6 TRAP Y
to:
In the following diagram showing Trapezoid Y, {$\bar{AC}$} and {$\bar{BD}$} are the legs, {$\bar{AB}$} and {$\bar{CD}$} are the bases, and {$\bar{EF}$} is the median.

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Trapezoid Y is an isosceles trapezoid because its legs are congruent ( = ). A trapezoid without congruent legs is simply called a trapezoid.
An isosceles trapezoid is symmetrical when cut at a right angle to the midpoint of the median.
The base angles of a trapezoid are the angles at the ends of its longer base, in Trapezoid Y’s case, and .

Let’s look at some basic principles.
Principle
1: The base angles of an isosceles trapezoid are congruent.
In the above diagram .

PIC
FIG 7 other trap
to:
Trapezoid Y is an isosceles trapezoid because its legs are congruent ({$\bar{AC}$} = {$\bar{BD}$}). A trapezoid without congruent legs is simply called a trapezoid.

An isosceles trapezoid is symmetrical when cut at a right angle to the midpoint of the median.

The
base angles of a trapezoid are the angles at the ends of its longer base, in Trapezoid Y’s case, {$\angle{C}$} and {$angle{D}$} .

!!Principles related to trapezoids
Let’s
look at some basic principles pertaining trapezoids.

!!!Principle
1:
%cframe
width=585px% '''The base angles of an isosceles trapezoid are congruent.
In the above diagram .'''
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Principle 2: Any trapezoid with congruent base angles is an isosceles trapezoid.
Thus, in the diagram used above, if , then .

Applying
Algebra to Trapezoid Figures
to:

!!!Principle
2:
%cframe
width=585px% '''Any trapezoid with congruent base angles is an isosceles trapezoid.
Thus, in the diagram used above, if , then .'''

!!
Applying Algebra to Trapezoid Figures
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%cframe width=585px%Principle 1: '''Interior angles of any quadrilateral will always add to exactly 360°.'''

%cframe width=585px%Principle 2: '''If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).'''
to:
!!!Principle 1:
%cframe width=585px%'''Interior angles of any quadrilateral will always add to exactly 360°.'''

!!!Principle 2:
%cframe width=585px%'''If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).'''
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%cframe width=495px% Attach:quad4.png"Quadrilaterals" | ''''-Figure 4: Since quadrilaterals have sum of its angles not more than 360 degrees, angles a and b must add up to 180.-''''

%cframe width=585px%Principle 3: '''Quadrilateral edges drawn to extend past corners will create supplementary interior/exterior angles.'''
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%cframe width=350px% Attach:quad4.png"Quadrilaterals" | ''''-Figure 4: Since quadrilaterals have sum of its angles not more than 360 degrees, angles a and b must add up to 180.-''''

!!!Principle 3:
%cframe width=585px%
'''Quadrilateral edges drawn to extend past corners will create supplementary interior/exterior angles.'''
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PIC FIG 4

Principle
3: Quadrilateral edges drawn to extend past corners will create supplementary interior/exterior angles.
to:
%cframe width=495px% Attach:quad4.png"Quadrilaterals" | ''''-Figure 4: Since quadrilaterals have sum of its angles not more than 360 degrees, angles a and b must add up to 180.-''''

%cframe width=585px%Principle 3: '''Quadrilateral edges drawn to extend past corners will create supplementary interior/exterior angles.'''
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PIC 3 FIG 5 1 quad with extending sides

Trapezoids
to:

!!Trapezoids
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%cframe width=585px% '''Principle 1: Interior angles of any quadrilateral will always add to exactly 360°.'''

%cframe width=585px% Principle 2: '''If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).'''
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%cframe width=585px%Principle 1: '''Interior angles of any quadrilateral will always add to exactly 360°.'''

%cframe width=585px%Principle 2: '''If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).'''
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Principle 1: Interior angles of any quadrilateral will always add to exactly 360°.

Principle
2: If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).
to:
%cframe width=585px% '''Principle 1: Interior angles of any quadrilateral will always add to exactly 360°.'''

%cframe
width=585px% Principle 2: '''If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).'''
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%cframe width=485px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=495px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=470px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=485px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=460px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=470px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=572px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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%cframe width=460px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''
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August 07, 2011 by matthew_suan -
August 07, 2011 by matthew_suan -
August 07, 2011 by matthew_suan -
A quadrilateral is a four-sided, two-dimensional enclosed figure (quad means four, lateral means side). Squares and rectangles are quadrilaterals, but quadrilaterals are not always square or rectangular. The sides do not have to be the same length, and can meet at any angle.

This section will look at angle relationships within quadrilaterals. Here are a few shapes that meet the requirements:

Here are a few that don’t. None of the following shapes are quadrilaterals. If you don’t understand why, slow down and have a think about each one. The first sentence in this article ought to help you out.

!!Sum of all angles in a quadrilateral
What do you think the angles of different quadrilaterals add up to? Why? Do you think quadrilaterals all have different angle sums, or does it depend on the length of their sides? (An angle sum is the total value of all interior angles in a shape.)
In the following diagram (see Figure 3), does A have the same angle sum as B?

%cframe width=572px% Attach:quad2.png"Quadrilaterals" | ''''-Figure 3: Totally different quadrilaterals A and B but their angle sums are just the same-''''

Think about cutting a square or rectangle in half diagonally…

If you already know that the angles of any and every triangle add to 180°, it should make sense to you that the angles of a quadrilateral sum to 360° (two times 180°).
This is easy to do with square and rectangular quadrilaterals, but harder when the sides are not all equal.

Later on in this article we’ll have a look at trapezoids. For now, let’s examine some principles related to quadrilaterals of a general nature.

Principle 1: Interior angles of any quadrilateral will always add to exactly 360°.

Principle 2: If two opposite angles in a quadrilateral are right angles, the other two opposite angles are supplementary (they add to 180°).

In the following diagram, angles a and b must add to 180° for the total enclosure to equal 360°.

PIC – FIG 4

Principle 3: Quadrilateral edges drawn to extend past corners will create supplementary interior/exterior angles.

In the following diagram, angles a and c are supplementary, and angles b and d are supplementary.

PIC 3 – FIG 5 – 1 quad with extending sides

Trapezoids

A trapezoid is a quadrilateral that has exactly two parallel sides. In other words, it has two parallel sides and two non-parallel sides. The parallel sides are called bases, the non-parallel sides connecting the bases are called legs. The median is the finite line segment connecting the two midpoints of the legs.
In the following diagram showing Trapezoid Y, and are the legs, and are the bases, and is the median.

PIC – FIG 6 – TRAP Y

Trapezoid Y is an isosceles trapezoid because its legs are congruent ( = ). A trapezoid without congruent legs is simply called a trapezoid.
An isosceles trapezoid is symmetrical when cut at a right angle to the midpoint of the median.
The base angles of a trapezoid are the angles at the ends of its longer base, in Trapezoid Y’s case, and .

Let’s look at some basic principles.
Principle 1: The base angles of an isosceles trapezoid are congruent.
In the above diagram .

PIC – FIG 7 other trap

Principle 2: Any trapezoid with congruent base angles is an isosceles trapezoid.
Thus, in the diagram used above, if , then .

Applying Algebra to Trapezoid Figures
In the following diagram, use your knowledge of geometry to find values for all four angles of each trapezoid by first finding values for x and y.

PIC – FIG 8
Question 1.
ABCD is an isosceles trapezoid.

PIC – FIG 9
Question 2.
As before, work out the angle values for each corner.

These shouldn’t be too hard for you to solve, just use a little logic and you’ll be fine.

Solutions are below!

Please note that a lower case x means ‘letter x’ which replaces a number, while upper case X denotes multiplication.
Question 1.

The statement that ABCD is isosceles has been given to us.
From this we can deduce two truths:
1. also = 5x + 60°.
2. 5x = 2x + y.
Therefore 5x – y = 2x.
Therefore y = 3x.
Working out the values for x and y is not too difficult from here. Because and are parallel, we know that = 180°.
In other words 10x + 60 = 180°.
Therefore 10x = 120°. (Deduct 60° from 180°.)
Therefore x = 120°/10
x = 12°.
Finding values for angles is as simple as substituting this value into x in the diagram.
= 5x = 5 X 12 = 60°
= 5x + 60 = (5 X 12) + 60 = 120°
Because ABCD is isosceles, we know and .

Question 2.

Solving the second diagram is a little more interesting. The fact that alone tells us that ABCD is not an isosceles trapezoid, so we’ll have to work out at least three angles in order to deduce the fourth.
We know that because and are parallel, we know that = 180°, and that = 180°.
We’ll work with and first. To put this mathematically,
12x – 15 + 15 = 180°.
Simplified, 12x = 180°.
Therefore x = 180/12 = 15°.
= 5 X 15 = 75°.
= 7 X 15 = 105°.
(A possibility to consider: These two values are equal to 180, therefore the lines are parallel, ABCD is indeed a trapezoid. If 180°, and are not parallel… and ABCD is not a trapezoid.)
= 9 X 15 = 135°.
There are two possible ways to find a value for .
The first is by deductive reasoning:
= 360 – ( + + ) = 360 – (75 + 105 + 135) = 45°.
To put this in words, must equal 360 minus the sum of the other three angles.
The second is by algebra:
We know that there are 24 fifteens in 360 ( 360/15 = 24). Because the other three angles sum to 21x, we know = 3x. We’ve been given a value of x + 2y for . From this we can conclude x = y because x + 2y = 3x.
= 3x = 3 X 15 = 45°.