Download When you classify a triangle, you need to be as specific as possible

Classifying Triangles
Triangles can be classified into two different groups: sides and angles.
When you classify a triangle, you need to be as specific as possible.
∆PAL has 3 acute angles and no congruent sides. It is an
A
acute scalene triangle.
79o
Note: Because none of the angle measures are 
on ∆PAL, then none of the side measures are  .
P
67o
34o
L
∆DRY has one obtuse angle and 2 congruent sides. It is an
obtuse isosceles triangle.
D
Note: Because two of the side measures are  on
∆DRY, then two of the angle measures are  .
4.1
Triangles and Angles
R
Y
Goal 1
Classifying Triangles
Each of the three points joining the sides of a triangle is a vertex. The plural
of vertex is vertices. In ∆FAN, points F, A, and N are vertices.
In a triangle, 2 sides sharing a common vertex are adjacent
sides. In ∆FAN, AF and AN are adjacent sides. The third side, FN
is the side opposite A.
A
79o
adjacent sides
F 67o
opposite
4.1
Triangles and Angles
34o
N
Goal 1
Right and Isosceles Triangles
The sides of right and isosceles triangles have special names.
In a right triangle, the sides that form the right angle are the legs of the right
triangle. The side opposite the right angle is the hypotenuse.
hypotenuse
leg
leg
When an isosceles triangle has only 2 congruent sides, then these 2
sides are the legs of the isosceles triangle. The third side is the base of the
isosceles triangle.
leg
base
leg
4.1
Triangles and Angles
Goal 1
Classifying Triangles
An isosceles triangle can have 3 congruent sides, in which case it is
equilateral. In ∆PAL, points P, A, and L are vertices.
An equilateral triangle has all 3 sides congruent. This means the 3
angles of the equilateral triangle have to be congruent to each other.
A
L
P
PA  AL
4.1
AL  LP
 P  A
 P  L
LP  PA
 A  L
Triangles and Angles
mP = 60o
mA = 60o
mL = 60o
The sum of the s = 180o