A Tale of Three Intersecting Lines
What a triangle is made of
Learn the parts that make up a triangle, and what to call them.
A triangle is the simplest closed shape. It is made of two kinds of thing:
- three corner points, the vertices
- three line segments, the sides, each joining two vertices
Triangles come in all shapes: tall and thin, wide and flat, perfectly even. They are all triangles, as long as they have three vertices and three sides.
We write a triangle with the symbol ∆ and name it by its vertices, in any order: ∆ABC, ∆BCA and ∆CAB are the same triangle.
Where two sides meet at a corner, they make an angle. So a triangle has three angles. In ∆ABC they are ∠CAB, ∠ABC and ∠BCA. Each is at a different corner, so we can shorten them to ∠A, ∠B and ∠C.
A question to keep in mind: what if the three points lie on one straight line? There are still three points and three segments, but the shape has gone flat and encloses nothing. That is not a triangle. This edge case matters later.
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