Classifying 2D Shapes — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. A regular polygon has all sides equal and all angles equal.
A regular polygon has all sides of equal length and all interior angles of equal measure. Examples: equilateral triangle, square, regular hexagon.
2. A triangle with sides 5 cm, 5 cm, and 8 cm is classified as isosceles.
An isosceles triangle has exactly two equal sides. With sides 5, 5, and 8, two sides are equal, so it is isosceles.
3. An acute triangle has 3 angles that are each less than 90°.
An acute triangle has all three angles less than 90°. If even one angle is 90° or more, the triangle is not acute.
4. A triangle with angles 30°, 60°, and 90° is classified as a right triangle.
A right triangle has exactly one angle measuring 90°. The 30-60-90 triangle is a commonly used special right triangle.
5. The third angle of a triangle with two angles of 45° each measures 90 degrees.
Angles sum to 180°. 45 + 45 = 90, so the third angle = 180 − 90 = 90°. This is a right isosceles triangle.
6. A triangle with sides 3 cm, 4 cm, and 5 cm is a scalene triangle by its sides.
A scalene triangle has no equal sides. Sides 3, 4, and 5 are all different, so it is scalene. (This is also a right triangle since 3²+4²=5².)
7. An equilateral triangle has three angles that each measure 60 degrees.
In an equilateral triangle, all three angles are equal. Since they sum to 180°, each angle = 180° ÷ 3 = 60°.
8. A pentagon is a polygon with 5 sides.
The prefix 'penta-' means five. A pentagon has 5 sides and 5 angles. A regular pentagon has all equal sides and angles of 108° each.
9. A triangle with one angle of 120° is called a(n) obtuse triangle.
An obtuse triangle has one angle greater than 90°. An angle of 120° is greater than 90°, so this is an obtuse triangle.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Three equal sides, all angles 60°
→ Equilateral triangle
Isosceles triangle
One angle exactly 90°
→ Right triangle
Equilateral triangle
No equal sides, no equal angles
→ Scalene triangle
Scalene triangle
Two equal sides, two equal base angles
→ Isosceles triangle
Right triangle
Three equal sides/60° = Equilateral; one 90° angle = Right triangle; no equal sides = Scalene; two equal sides = Isosceles.