Classifying Triangles — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. A triangle with sides of 5 cm, 5 cm, and 5 cm is called an equilateral triangle.
Three equal sides make the triangle equilateral, and each angle measures 60 degrees.
2. A triangle with sides of 7 in, 7 in, and 4 in is called an isosceles triangle.
Exactly two sides are equal, which is the definition of an isosceles triangle.
3. A triangle with sides of 3 cm, 4 cm, and 5 cm is called a scalene triangle.
All three sides are different lengths, so the triangle is scalene.
4. If a triangle has angles of 80, 60, and 40 degrees, the missing angle keeps the total at 180.
180 minus 80 minus 60 equals 40, so the third angle is 40 degrees.
5. A right triangle always has exactly one angle that measures 90 degrees.
Because the three angles must sum to 180 degrees, a triangle can have only one 90 degree angle.
6. A triangle with angles 110, 35, and 35 degrees is an obtuse isosceles triangle.
Two equal angles of 35 degrees make the triangle isosceles, and 110 degrees makes it obtuse.
7. A triangle that is both right and isosceles has angles 90, 45, and 45 degrees.
After the 90 degree right angle, the remaining 90 degrees splits into two equal 45 degree angles.
8. Angles 60, 60, 60 describe an equilateral triangle, which is also an acute triangle by angle.
All three angles are 60 degrees, which is less than 90, so the triangle is acute.
9. A triangle with angles of 95, 45, and 40 degrees is an obtuse triangle.
Because one angle, 95 degrees, is greater than 90 degrees, the triangle is obtuse.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Angles 60, 60, 60
→ Equilateral acute
Acute scalene
Angles 90, 45, 45
→ Right isosceles
Right isosceles
Angles 100, 40, 40
→ Obtuse isosceles
Equilateral acute
Angles 80, 60, 40
→ Acute scalene
Obtuse isosceles
Equal angles signal equal sides (isosceles or equilateral), and the largest angle decides acute, right, or obtuse.