Triangles (I): Introduction

1Summary 重點摘要

3 sides, 3 angles, 3 vertices.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Part 1: Three of each: 3 sides, 3 angles, 3 verticesA triangle has three features: 3 sides, 3 angles and 3 vertices. Draw three straight lines that join end to end, then count: exactly 3 sides, 3 angles and 3 vertices. This is a triangle's ID card.
2. Part 2: It must be closedA triangle must be closed. If you draw only two sides and stop, a gap is left in the middle, so it is not a triangle. The three sides must join end to end and enclose the shape; leaving out one side will not do.
3. Part 3: Triangles around usTriangles are everywhere in life: a set square, a cut corner of a sandwich, a yield road sign, the side of a tent, a triangle bell. When you see a closed shape made of three sides, you can call it a triangle.
4. Part 4: How to draw oneDrawing a triangle takes three steps. Step 1: use a ruler to draw a 6 cm base. Step 2: draw a slanting line from each end of the base. Step 3: let the two slanting lines meet at the top. Count 3 sides and 3 angles, and it is done.
5. Part 5: Triangles come in shapesTriangles come in different looks. An equilateral triangle has 3 equal sides; a right-angled triangle has one 90° corner, like a set square; some look long and flat. Their looks differ, but all have 3 sides and 3 angles.
6. Part 6: Compare with quadrilateralsCompare a triangle with a quadrilateral: a triangle has 3 sides and 3 angles, while a quadrilateral has 4 sides and 4 angles, like a square or a rectangle. Counting the sides is enough to tell them apart.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• A triangle has 3 sides, 3 angles and 3 vertices
• The 3 sides must be straight and closed
• Set squares, sandwich corners and signs are triangles
• Triangles may look different but always have 3 and 3
• Count sides and angles to tell triangles from quadrilaterals