A Tale of Three Intersecting Lines
Altitudes, and kinds of triangle
Learn what the altitudes of a triangle are, and the different kinds of triangle.
Another useful measurement: the height of each corner above the opposite side.
When we talk about the height of a person, a tree or a building, we mean measured straight up, at right angles to the ground, never a slanting distance.
In ∆ABC, draw AD from A perpendicular to BC. The length of AD is the height of A above BC. The segment AD is called an altitude. A triangle has three altitudes, one from each corner to the opposite side.
To draw one, place a set square along the side and slide it until its upright edge passes through the corner; then draw the line. (If the side is too short, extend it first.)
Kinds of triangle:
| By sides | |
|---|---|
| equilateral | all three sides equal |
| isosceles | two sides equal |
| scalene | no two sides equal |
| By angles | |
|---|---|
| acute | all three angles less than 90° (think cute: all small) |
| right | one angle exactly 90° |
| obtuse | one angle more than 90° |
The two ways of sorting are separate: a triangle can be isosceles and right-angled at the same time.
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