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Chapter 4.1 - Classifying Triangles
Objective: To be able to identify and classify
triangles by their angles and sides.
Check.4.9 Classify triangles, quadrilaterals, and
polygons (regular, non-regular, convex and
concave) using their properties.
Check.4.10 Identify and apply properties and
relationships of special figures (e.g., isosceles and
equilateral triangles, family of quadrilaterals,
polygons, and solids).
Classifying Triangles
Classify by Angles
• Acute Triangle
• All angles < 90
• Obtuse Triangle
• One angle > 90
Classify by Sides
• Scalene Triangle
• No two sides are the same
• Isosceles Triangle
• At least two sides are the
same
• Equilateral Triangle
• Right Triangle
• One angle = 90
• All sides are the same
Acute Triangles
Classify the Triangles
A
• ADB
• EDB
Right Triangles
B
C
D
•
•
•
•
ACD
ABC
DCE
BCE
Obtuse Triangles
E
• ADE
• AEB
Isosceles Triangles
Classify the Triangles
A
• ADB
• EDB
Equilateral Triangles
• none
B
C
E
Scalene Triangles
D
• AEB
• AED
• ACB
• ACD
• BCD
• DCE
Find Missing Values in Triangle
Find x and the measure of each side of the Equilateral
Triangle RST if RS = x + 9, ST = 2x, and RT = 3x -9
S
x+9
R
x + 9 = 2x
-x
-x
9 =x
2x
3x - 9
T
RS = x + 9
=9+9
= 18
Find Missing Values in Triangle
Find d and the measure of each side of the Equilateral
Triangle KLM if KL = d + 2, LM = 12 - d, and KM = 4d -13
L
d+2
K
12 - d
4d -13
M
d + 2 = 12 - d
+d
+d
2d + 2 = 12
-2
-2
2d = 10
d=5
KL = d + 2
=5+2
=7
Use the distance formula
• Find x and the measure of the sides of DEC. Classify
the triangle by its sides
E(-5, 3)
C (3, 9)
D (2, 2)
C
E
D
• Isosceles Triangle
Use the distance formula
• Find x and the measure of the sides of RST. Classify
the triangle by its sides
R(-1, -3)
S (8, -1)
T (4, 4)
T
R
• Scalene Triangle
S
RS = √85
ST= √41
RT= √74
Practice Assignment
• Block Page 239, 16 – 36 Every 4th and 44
• Honors page 239 30 – 52 Even

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