# Triangles and its all properties Contributed by: A triangle has three sides, angles, and vertices respectively. The total of all internal angles of a triangle is always equal to 180 degrees.
1. Triangles
Triangle
A triangle is a closed figure in a plane consisting of three
segments called sides. Any two sides intersect in
exactly one point called a vertex.
A triangle is named using the capital letters assigned to its vertices in a clockwise or
counterclockwise direction. For example, the triangle below can be named triangle ABC in a
counterclockwise direction starting with the vertex A.
a. What are other names for triangle ABC?
A triangle can be classified according to its sides, angles, or a combination of both. If a triangle
has three congruent sides, it is called an equilateral triangle as shown below.
A triangle with at least two sides congruent is called an isosceles triangle as shown below.
b. Are all equilateral triangles isosceles? Why or why not?
c. Are some isosceles triangles equilateral? Explain.
a. Triangle (∆) ACB, ∆BAC, ∆BCA, ∆CAB, ∆CBA
2. b. All equilateral triangles are also isosceles triangles since every equilateral triangle has
at least two of its sides congruent.
c. Some isosceles triangles can be equilateral if all three sides are congruent.
A triangle with no two of its sides congruent is called a scalene triangle and is shown below.
Classification of Triangles by Sides
Equilateral triangle: a triangle with three
congruent sides
Isosceles triangle: a triangle with at least two
sides congruent
Scalene triangle: a triangle with no two sides
congruent
Another way to classify triangles is according to their angles. A triangle with three acute angles
can be classified as an acute triangle.
A triangle with one obtuse angle can be classified as obtuse triangle.
3. A right triangle is a triangle with one right angle.
Segments PQ and RP are called the legs of the right triangle and segment RQ is called the
hypotenuse. The legs form the right angle ∠ RPQ. The side opposite the right angle is
hypotenuse RQ.
Classification of Triangles by Angles
Acute triangle: a triangle with three acute angles
Obtuse triangle: a triangle with one obtuse angle
Right triangle: a triangle with one right angle
True or False: Give a reason or counterexample to justify your response.
1. An equilateral triangle is always acute.
2. An obtuse triangle can also be isosceles.
3. The acute angles of a right triangle are complementary.
4. Use the figure below and find the value of x for each of the following.
4. a) AC= (x2-2x+4) and BC= (x2+3x-11).
b) BC= 17+3x and AC= x+25
c) AC= x2-6x and BC= x-12
5. Given ∆ABC with vertices A(1,5), B(5,5), and C(5,1)
a) graph ∆ABC in the coordinate plane.
b) classify this triangle by its sides and angles.
Triangles can also be classified by using a combination of angle and side descriptors.
Right isosceles triangle
Right scalene triangle
Obtuse isosceles triangle
5. Complete each statement below with always, sometimes, or never and give a justification for
1. A scalene triangle is _________ an acute triangle.
2. A right triangle is __________ an obtuse triangle.
3. An isosceles triangle is_________ a right triangle.
4. An equilateral triangle is __________ an isosceles triangle.
5. The acute angles of a right triangle are________ supplementary.
6. A right isosceles triangle is _________ equilateral.
6. Exploration
Using linguine, snap off the ends to make segments 3, 5, 6, and 9 inches long.
1. Determine which sets of three lengths will make a triangle.
2. Which sets of three segments did not form a closed figure in the plane?
3. What do the sets that form a triangle have in common?
1. A triangle can be formed using the following sets of lengths:
3, 5, 6 5, 6, 9
2. The set consisting of 3, 6, and 9 did not form a triangle. 3+6=9
3. The sum of the lengths of any two sides of a triangle is greater than the length
of the third side.
This exploration leads to the following theorem:
Triangle Inequality Theorem
The sum of the lengths of any two sides
of a triangle is greater than the length of
the third side.
1. Two sides of a triangle have lengths of 4 cm and 7 cm. What are the possible
lengths for the third side?
4 cm + 7 cm > x and 4 cm + x > 7 cm and 7 cm + x > 4 cm
11 cm > x and x > 3 cm and x > -3 cm
The intersection of these inequalities can be represented graphically as the
intersection of three rays with open endpoints as shown below.
7. All possible lengths of the third side are represented by the inequality
11cm > x > 3 cm.
1. The lengths of three segments are given. Determine if these segments can be
used to form a triangle.
a) 11 cm, 15 cm, and 23 cm
b) 7.5 in, 8.3 in, and 4.2 in
2. The lengths of two sides of ∆ABC are given as A=12 ft and BC=17 ft.
What are the possible lengths of the third side AC?
3. ∆DEF has side lengths as follows: DF=(x+1) m , DE=(3x-4) m , and
EF= (x+7) m. What are the possible values of x ?
Segments of Triangles
We will discuss three segments in a triangle: altitudes, medians, angle bisectors
Definition
An altitude of a triangle is the segment drawn from
a vertex perpendicular to the opposite side or
extension of that side.
8. Every triangle has three altitudes as shown in the figures below.
In the previous drawings, it seems that the altitudes intersect in a common point. Investigate this
idea by using paper folding with patty paper.
a) Draw a large triangle on a sheet of patty paper.
b) Cut out the triangle along its sides.
c) Fold the altitudes of this triangle.
d) The common point of intersection of these altitudes is called the orthocenter.
Definition
A median of a triangle is a segment having
one endpoint at a vertex of a triangle and
the other endpoint at the midpoint of the
opposite side.
A triangle also has three medians as shown in the diagram below.
The medians in the drawing also seem to meet in a common point. Use patty paper and paper
folding to verify this idea.
a) Draw a large triangle on a sheet of patty paper.
b) Cut out the triangle along its sides.
9. c) Crease each segment in the middle after matching its endpoints by folding the
paper. This point that divides each segment into two congruent segments is called
a midpoint.
d) Make another fold connecting the midpoint of a side with the opposite vertex
to form the median. Repeat this process for the other two sides.
e) The point where all three medians intersect is called the centroid or center of
mass.
Definition
An angle bisector of a triangle is the segment
that bisects an angle of a triangle with one
endpoint at the vertex of the angle bisected
and the other endpoint on the opposite side
of the triangle.
Every triangle has three angle bisectors as shown in the figure below.
We have medians and altitudes intersecting in a common point and it seems that the angle
bisectors also have a common point of intersection. Use paper folding with patty paper to
investigate this idea.
a) Begin by drawing a large triangle on a sheet of patty paper.
b) Use scissors to cut out the triangle along its sides.
c) Hold an angle at its vertex and fold so that the sides meet along a line that
includes the vertex. Continue this process and fold the other angle bisectors.
d) The common point of intersection of these angle bisectors is called the incenter,
the center of the inscribed circle in the triangle.
10. It has been shown that the altitudes, medians, and angle bisectors each have a common point of
intersection called a point of concurrency.
1. Given: ∆DOT as shown
Find the value of x so that AT is an altitude.
2. Given: ∆PQR as shown
PM = (3x- 8) in
MR = (x + 5) in
Find the value of x so that RM is a median.
3. Given: ∆ANG as shown below
m∠NAB = (5x – 4)
m∠GAB = (3x + 10)
Find: x so that AB is the angle bisector of ∠NAG
11. Solutions:
1. TA ⊥ DO (Definition of an altitude)
∠TAD is a right angle (⊥s form right angles)
m∠TAD = 90 (Right ∠s have a measure of 90°.)
3x + 15 = 90 (Substitution)
3x = 75 (Subtraction property of equality)
x = 25 (Division/multiplication property of equality)
2. PM = MR (Definition of a median)
3x – 8 = x + 5 (Substitution)
2x = 13 (Addition property of equality)
x = 6.5 (Division/multiplication property of equality)
3. m∠NAB = m∠GAB (Definition of an angle bisector)
5x – 4 = 3x + 10 (Substitution)
2x = 14 (Addition property of equality)
x=7 (Division/multiplication property of equality)
1. Given: ∆SWI
1
SM = ( x + 3) cm
2
2
MW = ( x – 1) cm
3
12. Find: x so that IM is a median
2. Given: ∆ABC
m∠ABD = (5x – 7.5)
m∠CBD = (3x + 16.5)
Find: x so that BD is an angle bisector
3. Given: ∆CAN
m∠ATN = (4x + 18)
Find: x so that AT is an altitude
13. Congruent Triangles
Cut pieces of linguine into lengths of 6 in, 8 in, and 10 in.
1. Use the pieces of linguine to form a triangle.
2. Is it possible to form a different triangle using these lengths? Explain.
3. How do these triangles compare?
1. The pieces of linguine can be used to form the following triangle.
2. It is possible to form triangles with different orientations in the plane as shown
below.
3. The triangles have the same size and shape as the original triangle shown.
Use a piece of tracing or patty paper to trace the triangles in solution 2. Use rotations and
translations to match corresponding sides .
1. How do the corresponding angles compare?
2. How many parts of one triangle match with corresponding parts of another triangle
having the same size and shape?
3. What is the relationship between corresponding sides and corresponding angles in the
set of triangles?
1. The corresponding angles have the same measure.
2. Three sides and three angles of one triangle match with three corresponding sides and
three corresponding angles of another triangle.
3. Corresponding sides are opposite corresponding angles. The triangles
in solution 1 and solution 2 are said to be congruent.
14. Congruent Triangles
Two triangles are congruent if and only
if their corresponding sides and their
corresponding angles are congruent.
1. Given triangle ABC is congruent to triangle DEF. Identify the corresponding parts in
the two triangles.
Another way to state that triangle ABC is congruent to triangle DEF is by using the
notation: ∆ABC ≅ ∆DEF
The corresponding sides and corresponding angles can be identified by matching the
corresponding vertices of the two triangles as shown below.
The corresponding sides and corresponding angles of two congruent triangles are referred to as
corresponding parts of congruent triangles. We often write CPCTC for “Corresponding Parts of
Congruent Triangles are Congruent”.
15. 2. Show that the congruence of triangles is reflexive.
Given: ∆RST
Show ∆RST ≅ ∆RST
We know that RT ≅ RT , RS ≅ RS , and TS ≅ TS by the reflexive property of
equality. ∠R ≅ ∠R, ∠S ≅ ∠S, and ∠T ≅ ∠T by the reflexive property of equality.
We now have three sides and three angles of ∆RST congruent to the three
corresponding sides and corresponding angles of ∆RST. By the definition of
congruent triangles, ∆RST ≅ ∆RST.
3. Show that the congruence of triangles is transitive.
Given: ∆RST ≅ ∆UVW, ∆UVW ≅ ∆XYZ
Show ∆RST ≅ ∆XYZ
We know that RS ≅ UV , RT ≅ UW , ST ≅ VW , ∠R ≅ ∠U, ∠S ≅ ∠V, and
∠T ≅ ∠W by CPCTC given ∆RST ≅ ∆UVW. Since ∆UVW ≅ ∆XYZ, we have
UV ≅ XY , UW ≅ XZ , VW ≅ YZ , ∠U ≅ ∠X, ∠V ≅ ∠Y, and ∠W ≅ ∠Z by
CPCTC. We know that congruent sides have equal measures and by the transitive
property of equality we have the following: RS = XY, RT = XZ, and ST =YZ
Using a similar justification, the following is true for corresponding angles:
m∠R = m∠X, m∠S = m∠Y, and m∠T = m∠Z
∆RST ≅ ∆XYZ by definition of congruent triangles.
4. Congruence of triangles is also symmetric. This justification is left as an optional
exercise.
16. 1. Given ∆MNO ≅ ∆PQR, identify the corresponding parts.
2. Given: ∆MNO ≅ ∆PQR, MN = (5x-11), PQ = (3x-2)
Find: x and MN
3. Given: ∆KLM ≅ ∆NOP, m∠M = (5x + 23), and m∠P = (3x + 40)
Find: x and m∠P
4. Given: ∆ABC with vertices A(-3,2), B(2,6), and C(-3,6) and
∆EFG with vertices E(2,2), F(7,-2), and G(2,-2)
a) Find the lengths of each side and determine if the triangles are congruent.
b) If the triangles are congruent, write a congruence statement for the two triangles.
If the triangles are not congruent, justify your conclusion.
By the definition of congruent triangles, two triangles are congruent if and only if the six parts of
one triangle are congruent to the six corresponding parts of a second triangle. Is it possible for
two triangles to be congruent using only some of the corresponding parts? If so, which
corresponding parts are sufficient to show two triangles congruent?
Do problem 1 in the activity Look Alikes using Cabri Junior™ on a TI-83+ graphing calculator.
In this exploration, a triangle will be constructed using three sides of a given triangle. The results
of this exploration leads us to the SSS congruence postulate for proving two triangles congruent.
SSS Congruence Postulate
Two triangles are congruent if and only if
three sides of one triangle are congruent
to three sides of a second triangle.
1. Determine which of the following represents a pair of congruent triangles.
17. 2. Given: ∆ACB is isosceles , point M is the midpoint of AB
Show ∆AMC ≅ ∆BMC.
AC ≅ CB by the definition of an isosceles triangle. AM ≅ MB since M is the
midpoint of AB . CM ≅ CM by the reflexive property of equality. Now we have
∆AMC ≅ ∆BMC by the SSS congruency postulate.
3. Given: RS ≅ VT , RT ≅ SV
Prove: ∠SRT ≅ ∠TVS
Statements Reasons
1. RS ≅ VT 1. Given
RT ≅ SV
2. ST ≅ ST 2. Reflexive property of equality
3. ∆SRT ≅ ∆TVS 3. SSS congruence postulate
4. ∠SRT ≅ ∠TVS 4. CPCTC
18. 1. Given: ∆MNO ≅ ∆PQR
Determine whether each statement is true or false and give a justification for your
a) MN ≅ PQ
b) ∠MNO ≅ ∠PRQ
c) m∠OMN = m∠QPR
2. Given: PQ ≅ SR
PS ≅ QR
Prove: ∆PQS ≅ ∆RSQ
Complete the proof with an appropriate statement or reason.
Statements Reasons
1. PQ ≅ SR 1. _________________________
PS ≅ QR
2. ___________________________ 2. Transitive property of equality
3. ∆PQS ≅ ∆RSQ 3. __________________________
19. 3. Given: AE ≅ BF
EC ≅ DF
AB ≅ CD
Prove: ∠AEC ≅ ∠BFD
Proof:
Statements Reasons
1. AE ≅ BF 1.____________________________
EC ≅ DF
AB ≅ CD
2. AE = BF, EC = DF, AB = CD 2. ___________________________
3. _________________________ 3. Reflexive property
4. AB + BC = CD + BC 4. ___________________________
5. AB + BC = AC, CD + BC = BD 5. ___________________________
6. __________________________ 6. Substitution
7. __________________________ 7. SSS congruence postulate
8. ∠AEC ≅ ∠BFD 8. ___________________________
4. Given: ∆RST with vertices R(-3,8), S(2,5), and T(2,8)
∆MNP with vertices M(10,0), N(5,3), and P(5,0)
20. Show ∆RST ≅ ∆MNP.
Refer to the activity Look Alikes in the Geometry Module. Use a TI-83+ graphing calculator
with Cabri Junior™ and do problem 2. In this problem, a triangle will be constructed using two
sides and the included angle of a given triangle. Side lengths and angle measures of the
constructed triangle will be compared to the corresponding side lengths and angle measures of
the original triangle.
This activity refers to two sides and the included angle of a triangle. In the diagram below, the
sides XZ and XY of ∆XYZ are included in the sides of ∠ZXY
In the diagram below, the given sides (marked) are not included in the sides of ∠ZXY. This
diagram does not represent two sides and the included angle of a triangle.
The results of this exploration lead us to the following congruence postulate for triangles.
21. SAS Congruence Postulate
Two triangles are congruent if and only if
two sides and the included angle of one
triangle are congruent to two sides and the
included angle of the second triangle.
1. Which of the following represents a pair of congruent triangles? State the congruence
postulate that applies and write a justification.
22. d) Given: AD and CE bisect each other at point O
+ AOC ≅+ DOE
1. a) The triangles do not represent a pair of congruent triangles since the angle shown
is not the included angle for the two sides given.
b) The pair of triangles shown are congruent by the SAS congruence postulate since
the sides marked are included in the sides of the right angle.
c) ∆RST ≅ ∆RUT by the SAS congruence postulate as follows:
S ST ≅ TU These corresponding sides are given.
A ∠STR ≅ ∠RUT An angle bisector divides an angle into two congruent
angles.
S RT ≅ RT Reflexive property
d) ∆AOC ≅ ∆DOE by the SAS and SSS congruence postulates.
Using the SAS congruence postulate, we have the following justification:
S CO ≅ EO A bisector of a segment divides it into two congruent
segments.
A ∠AOC ≅ ∠DOE Vertical angles are congruent.
S AO ≅ DO A bisector of a segment divides it into two congruent
segments.
Using the SSS congruence postulate, we have the following justification:
S AC ≅ DE Given
S CO ≅ EO A bisector of a segment divides it into two congruent
segments.
S AO ≅ DO A bisector of a segment divides it into two congruent
segments.
23. 2. Given: AC ≅ BD
∠CAB ≅ ∠DBA
Complete the following proof.
Statements Reasons
1. AC ≅ BD 1. __________________________
∠CAB ≅ ∠DBA
2. _________________________
2. Reflexive property
3. _________________________
3. SAS congruence postulate
4. __________________________
3. Given: ∆ABC with vertices A(-2,6), B(1,2), and C(1,8)
∆DFE with vertices D(7,4), F(4,0), and E(4,6)
∠BAC = ∠FDE = 86.8°
a) Graph the coordinates of the vertices of each triangle in the coordinate plane.
24. b) Determine if ∆ABC ≅ ∆DFE and state the congruence postulate used.
4. Given: ∆KAR ≅ ∆GTO with KA = (3x-10) , GT = (x+2),
m∠KAR = (5y-40) and m∠GTO = (3y-8)
Find: x, y and KA
2. Statements Reasons
1. AC ≅ BD 1. Given
∠CAB ≅ ∠DBA
2. Reflexive property
2. AB ≅ AB
3. SAS congruence postulate
3. ∆CAB ≅ ∆DBA
4. AD ≅ BC 4. CPCTC
3. b) The distance formula can be used to determine the distance between two points
with coordinates (x, y) and (x1,y1) in the coordinate plane:
d = ( x − x1 ) 2 + ( y − y1 ) 2
Using the distance formula above, the following lengths of sides can be
determined.
AC = (−2 − 1) 2 + (6 − 8) 2 = (−3) 2 + (−2) 2 = 9+4 = 13
AB = (−2 − 1) 2 + (6 − 2) 2 = (−3) 2 + 4 2 = 9 + 16 = 25 = 5
ED = (4 − 7) 2 + (6 − 4) 2 = (−3) 2 + 2 2 = 9+4 = 13
25. DF = (7 − 4) 2 + (4 − 0) 2 = 32 + 4 2 = 9 + 16 = 25 = 5
We now have AC ≅ DE and AB ≅ DF since congruent segments have equal
measures. ∠BAC and ∠FDE have the same measure and are also congruent.
This gives us two sides and an included angle of ∆ABC congruent to two sides
and an included angle of ∆DFE. Therefore, ∆ABC ≅ ∆DFE by the SAS
congruence postulate.
4. We have KA = GT from the congruence statement. This allows us to write and solve
the following equation:
3x -10 = x + 2
2x = 12
x= 6
To find KA, substitute the value 6 for x in the equation KA = (3x-10) and get
KA = (3•6-10) = 8.
Since corresponding angles of congruent triangles are congruent,
m∠KAR = m∠GTO. By substituting the algebraic expressions for these measures,
we have the following equation: 5y-40 = 3y-8
Solve this equation for y : 2y = 32
y = 16
1. Given: AC ≅ BC
CD ≅ CE
Prove: ∠A ≅ ∠B
26. Complete the following proof.
Statements Reasons
1. ___________________________ 1. Given
___________________________
2. ___________________________ 2. Reflexive property
3. ∆ACE ≅ ∆BCD 3. __________________________
4. ___________________________ 4. CPCTC
2. Given: ∆ACB is isosceles with base AB
ray CD bisects ∠ACB
Write a justification explaining why ∆ADB is isosceles. This justification may be
written in the two-column format like the proof in exercise 1 or in a paragraph format.
3. Given: ∆PQR with vertices P(-3,1), Q(1,1), and R(1,6)
∆STU with vertices S(8,2), T(4,2), and U(4,7)
a) Graph the triangles in the coordinate plane.
b) Determine if ∆PQR and ∆STU are congruent.
4. Given: Right triangle ACB with right ∠ACB
Right triangle DEF with right ∠DEF
AC ≅ DE and BC ≅ EF
Prove: ∆ACB ≅ ∆DEF
This proof leads to the following theorem for proving right triangles congruent.
27. Leg-Leg Theorem
If two legs of one right triangle are
congruent to two legs of a second right
triangle, then the right triangles are
congruent.
5. Given: Circle O with radii OG and OH
OM bisects ∠GOH
GM = x2 + 2x + 5
HM = x2 + x + 11
OM = 3x – 6
Find: x, GH, and OM
Refer to the activity Look Alikes in the Geometry Module. Use a TI-83+ graphing calculator
with Cabri Junior™ and do problem 3. In this problem, a triangle will be constructed using two
angles and the included side of a given triangle. Angle measures and side lengths of the
constructed triangle will be compared to the corresponding angle measures and side lengths of
the original triangle.
This activity refers to two angles and the included side of a triangle. In the diagram below, HI
is the included side between ∠H and ∠I.
28. The results of this exploration lead us to the following congruence postulate for triangles.
ASA Congruence Postulate
Two triangles are congruent if and only if
two angles and the included side of one
triangle are congruent to two angles and
the included side of a second triangle.
1. Which of the following pairs of triangles are congruent by the ASA congruence
2. Given: CD bisects ∠ACB
CD ⊥ AB
29. Prove: ∆ACB is isosceles
using a flow-chart proof.
∠ABD ≅ ∠CDB
m ∠A = 3x + 15
m ∠C = 8x – 20
Find: x and m ∠A
1. a) ∆CAM ≅ ∆DBM by the ASA congruence postulate.
A ∠A ≅ ∠B (Right angles are congruent.)
S AM ≅ MB (A midpoint divides a segment into two congruent
segments.)
A ∠AMC ≅ ∠BMD (Given)
b) ∆WOX ≅ ∆ZOY by the SAS congruence postulate. The ASA congruence
postulate does not apply to this problem.
S XO ≅ YO (A bisector divides a segment into two congruent
segments.)
30. A ∠XOW ≅ ∠YOZ (Vertical angles are congruent.)
S WO ≅ OZ (A bisector divides a segment into two congruent
segments.)
2. A flow-chart proof shows the logical development of a proof using statements with
supporting justification(s) in a flow-chart format. The statements and reasons are
shown in rectangles with arrows indicating how they connect to other information
in rectangles. The flow of information should lead to the conclusion. The proof
that follows models one format that can be used.
CD bisects ∠ACB ∠ACD≅∠BCD
Definition of an
Given angle bisector
∆ACB is
isosceles
CD ≅ CD ∆ADC≅∆BDC AC ≅ BC Definition of
ASA postulate
Reflexive CPCTC an isosceles ∆
Given ⊥s form rt. ∠s All right ∠s≅
3. ∆ADB ≅ ∆CBD by the ASA congruence postulate.
S DB ≅ DB (Reflexive property)
A ∠ABD ≅ ∠CDB (Given)
∠A ≅ ∠C (CPCTC)
m ∠A = m ∠C (Congruent angles have equal measures.)
3x+15 = 8x-20 (Substitution)
35 = 5x (Addition and subtraction properties of equality)
7=x (Division property of equality)
m ∠A = 3x + 15 = 3•7 + 15 = 21 + 15 = 36
31. 1. Given: AD bisects BC
AB ⊥ BC
DC ⊥ BC
Proof:
Complete the proof below.
Statements Reasons
1. AD bisects BC 1. __________________________
AB ⊥ BC
DC ⊥ BC
2. ∠B and ∠C are right angles 2. __________________________
3. _________________________ 3. All right angles are congruent.
4. BE ≅ BC 4. __________________________
5. ∠AEB ≅ ∠DEC 5. __________________________
6. _________________________ 6. ASA congruence postulate
7. _________________________ 7. CPCTC
8. BC bisects AD 8. __________________________
32. 2. Given: ∆YVW is isosceles with base VW
∠UYV ≅ ∠XYW
∠UVY ≅ ∠XWY
Prove ∠U ≅ ∠X using a flow-chart proof.
3. Given: ∠BAC ≅ ∠DCA
AE ≅ EC
DC = (x2 + 2x – 1) cm
AB = (x2 + 5x – 7) cm
Find: x and AB
4. Given: ∠M ≅ ∠R
MI ≅ RI
m ∠RPI = (9x – 10)
m ∠MQI = (4x + 45)
33. Find: x and m ∠MQI
We have explored the SSS, SAS, and ASA congruence postulates and LL theorem (right
triangles only). Are there other ways to show two triangles congruent? We will consider the
possibility of using any two angles and a side (not included) of one triangle (AAS) congruent to
the corresponding parts of another triangle in the following proof.
Given: ∆HIJ and ∆KLM
∠H ≅ ∠K, ∠J ≅ ∠M
HI ≅ KL
Prove: ∆HIJ ≅ ∆KLM
34. ∆HIJ and ∆KLM
Given
∠H ≅ ∠K ∆HIJ ≅ ∆KLM
Given ∠I ≅ ∠L
If 2 ∠s of one ∆ ≅ 2 ASA
∠s of a second ∆, congruence
∠J ≅ ∠M 3rd ∠s are ≅. postulate
Given
HI ≅ KL
Given
This proof leads to the following theorem for showing two triangles congruent.
AAS Congruence Theorem
If two angles and a side (not included)
are congruent to the corresponding parts
of a second triangle, then the two
triangles are congruent.
Is it possible for two triangles to be congruent using SSA where the angle is not included
between the two sides? Make a conjecture: ____________________________________
_______________________________________________________________________.
This conjecture will be tested in the following exploration.
Given: Two segments and a non-included angle
35. Patty paper, a compass, and straight edge will be needed for this exploration. Follow the
procedure outlined below.
a) Trace the longer segment given ( AB ) on a sheet of patty paper.
b) Place the patty paper over the given angle and align the vertex of the angle with
endpoint A of the longer segment. Align one of the sides of the angle with AB .
c) Trace the angle and extend the side not aligned with the segment as shown below.
d) Construct circle B with a radius equal to the length of the shorter segment given.
( The construction below is not drawn to scale.)
Circle B intersects ray AD in two points, point C and point D. There are two possible
triangles that can be constructed with the given ∠CAB, AB , and the shorter segment
represented by the radii BC and BD . ∆ABC and ∆ABD are the two triangles shown
in the figure above. Therefore, it is not possible to construct a unique triangle given
two sides and a non-included angle.
36. 1. Given: ∆CED is isosceles with base CD
CF ⊥ GE
CH = ED
CE = AG
∠A ≅ ∠FEC
∠GBF ≅ ∠CFB
Identify all pairs of congruent triangles in the figure given and explain the congruence
postulate or theorem used.
2. Given: ∠1 ≅ ∠2
DB ⊥ AC
37. Write a paragraph, two-column, or flow-chart proof.
3. Given: ∆DBE is isosceles with base DE
∠A ≅ ∠C
∠ABE ≅ ∠CBD
1
AB = ( x + 1) cm
2
3
BC = ( x – 5) cm
4
Find: x, AB, and BC
1. ∆ABG ≅ ∆EFC by the AAS congruence theorem
A ∠A ≅ ∠FEC (Given)
A ∠ABG ≅ ∠EFC (⊥s form rt. ∠s and all rt. ∠s ≅)
S AG ≅ CE (CE=ED=AG; ≅ seg. have = meas.)
∆CDE ≅ ∆CDH by the SSS congruence postulate
S CH ≅ DE (Given; ≅ seg. have = meas.)
S CE ≅ DH (CE=ED=CH=DH; ≅ seg. have = meas.)
S CD ≅ CD (Reflexive property)
∆GBF ≅ ∆CFB by the SAS congruence postulate
S GB ≅ CF (∆ABG ≅ ∆EFC; CPCTC)
38. A ∠GBF ≅ ∠CFB (Given)
S BF ≅ BF (Reflexive property)
2. Triangle ABD will be shown congruent to triangle CBD using the AAS congruence
theorem in a paragraph-style proof as follows.
Angle 1 is congruent to ∠DAB and ∠2 is congruent to ∠DCB because vertical angles
are congruent. Angle DBA and ∠DBC are right angles since DB ⊥ AC . It follows
that ∠DBA ≅ ∠DBC because all right angles are congruent. Side DB is congruent to
itself by the reflexive property. We now have ∆ADB ≅ ∆CDB by the AAS
congruence theorem. Angle ADB is congruent to ∠CDB by CPCTC. Therefore, DB
3. ∆ABE ≅ ∆CBD by AAS
A ∠A ≅ ∠C (Given)
A ∠ABE ≅ ∠CBD (Given)
S BE ≅ BD (An isosceles triangle has 2 congruent sides.)
AB ≅ BC (CPCTC)
AB = BC (Congruent segments have equal measures.)
1 3
x+1= x–5 (Substitution)
2 4
1
6= x
4
24 = x
1
AB = •24 + 1 = 13 cm CD = AB = 13 cm
2
1. Given: ∠BAE ≅ ∠DBA
DB ≅ AE
a) Identify all pairs of congruent triangles in the given figure.
b) Explain the congruence postulate or theorem used.
39. 2. Given: ∠RVS ≅ ∠USV
∠RSV ≅ ∠UVS
∠WRV ≅ ∠TUS
∠W ≅ ∠T
2
VW = ( x + 3) in
3
5
ST = ( x – 1) in
6
1
RS = ( x + 5) in
2
3. Given: Isosceles ∆ABC with base AB
CD bisects the vertex angle
Prove: CD is a median to the base.
Write a paragraph, two-column, or flow-chart proof.
40. This proof leads to the following theorem for isosceles triangles.
Isosceles Triangle Theorem
The bisector of the vertex angle of an
isosceles triangle is the same segment
as the median to the base.
4. Given: ∆FRD with vertices F(-4,1), R(-1,3), and D(-3,5)
∆TSU with vertices T(1,1), S(4,3), and U(2,5)
∠F ≅ ∠T and ∠D≅ ∠U
a) Graph each triangle in the coordinate plane.
b) Show ∆FRD ≅ ∆TSU.
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