Master regular & irregular polygons, quadrilaterals, and classifying triangles. This lesson covers SEA Mathematics Objectives 109-112, 114-116 from the Geometry Strand.
Did you complete Lesson 2.1? Review Solids & Plane Shapes →
By the end of this lesson, you will be able to:
Geometry questions appear in Section I and II of the SEA Math paper. Know your shape properties! (SEA Framework Page 23-24)
Start by watching this video to learn about regular and irregular polygons. Take notes as you watch! 📝
Did you understand the video? Review the notes below before continuing! ✅
A polygon is a closed 2D shape with straight sides. Let’s learn the difference between regular and irregular polygons!
| Type | Definition | Example |
|---|---|---|
| Regular Polygon | All sides EQUAL and all angles EQUAL | Regular hexagon, square |
| Irregular Polygon | Sides and/or angles are NOT all equal | Irregular pentagon, rectangle |
3 sides
3 vertices
180° total angles
4 sides
4 vertices
360° total angles
5 sides
5 vertices
540° total angles
6 sides
6 vertices
720° total angles
Regular: All sides and angles equal (like a square)
Irregular: Sides and/or angles different (like a rectangle)
Now watch this video to learn about different types of quadrilaterals. Pay attention to their properties! 🎯
All quadrilaterals have 4 sides and 4 angles that add up to 360°. Let’s learn the different types!
4 equal sides
4 right angles
2 pairs parallel sides
2 pairs equal sides
4 right angles
2 pairs parallel sides
4 equal sides
Opposite angles equal
2 pairs parallel sides
2 pairs equal sides
Opposite angles equal
2 pairs parallel sides
4 sides
1 pair parallel sides
Angles vary
2 pairs adjacent equal sides
1 pair opposite angles equal
No parallel sides
| Quadrilateral | Equal Sides | Right Angles | Parallel Sides |
|---|---|---|---|
| Square | 4 equal | 4 right angles | 2 pairs |
| Rectangle | 2 pairs equal | 4 right angles | 2 pairs |
| Rhombus | 4 equal | None (usually) | 2 pairs |
| Parallelogram | 2 pairs equal | None (usually) | 2 pairs |
| Trapezium | None (usually) | None (usually) | 1 pair |
| Kite | 2 pairs adjacent | None (usually) | None |
A square is ALSO a rectangle and a rhombus! All squares are rectangles, but NOT all rectangles are squares!
Watch this video to learn how to classify triangles by sides and angles. 🎯
Triangles can be classified by their SIDES or by their ANGLES. All triangles have angles that add up to 180°!
NO equal sides
NO equal angles
All sides different
2 equal sides
2 equal angles
1 line of symmetry
3 equal sides
3 equal angles (60°)
3 lines of symmetry
1 right angle (90°)
2 acute angles
Can be isosceles or scalene
All 3 angles acute
All angles < 90°
Can be any side type
1 obtuse angle
2 acute angles
Obtuse > 90°
Scalene: All sides different (like a scale – uneven)
Isosceles: 2 sides equal (eye-so-seles – 2 eyes equal)
Equilateral: All sides equal (equi = equal)
Test your understanding! Select an answer for each question. 🔷
Great job finishing Lesson 2.2! You’re mastering geometry for the SEA exam! 🎉
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