Page 71 - Math Course 3 (Book 2)

Identifying Triangles: Angles and Sides

Key Concept

Classifying Triangles by Angle Classifying Triangles by Sides

In an acute In an obtuse In an right No two sides At least two All sides of the
triangle, all of triangle, one triangle, one of a scalene sides of an sides of an
the angles are angle is obtuse. angle is right triangle are isosceles equilateral
acute. congruent triangle are triangle are
congruent. congruent.

42°
67°
13°
142° 25°
37° 76°
90° 48°

all angle one angle one angle
measures < 90 measures > 90 measures = 90

Let’s Begin

Classify Triangles by Angles Classify Triangles by Sides

Example Example

Identify the isosceles triangles in the figure if
ARCHITECTURE
The triangle truss below is modeled for steel con- UV ≅ VX ≅ UX.
struction. Use a protractor to classify ΔJMN, ΔJKO,
and ΔOLN as acute, equiangular, obtuse, or right. V
M
W
Y Z
L T

K

J
O N

U X

Answer Isosceles triangles have at least two sides
congruent.
ΔJMN has one angle with measure greater than
90, so it is an obtuse triangle. ΔJKO has one
angle with measure equal to 90, so it is a right
triangle. ΔOLN is an acute triangle with all
angles congruent, so it is an equiangular Answer ΔUTX and ΔUVX are isosceles.
triangle.

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