Meet the triangle family
Every angle has one side that is open. If you close this open side,
you’ll have a closed shape with three straight sides and three angles.
A shape of this kind is called a triangle [(try]{.smallcaps} ang guhl).
Triangle means “three angles.”
If a triangle could talk, it might say that it’s “better” than either a
point or a line. A point, remember, has no dimensions. A line has one
dimension—length. But a triangle (or any other closed shape, such as a
square or circle), has two dimensions—length and width. You can
measure a triangle’s sides to find out how long they are. And you can
measure the distance from one side to another side to find out how wide
a triangle is.
Just as there are different kinds of angles, there are different kinds
of triangles. And just as angles have special names, so do triangles.
Some triangles are named after the kinds of angles they have.
For example, an angle that makes a square corner is called a right
angle. And a triangle with a right angle in it is called a right
triangle.
An angle that is less than a right angle is called an acute angle. And a
triangle with three acute angles is called an acute triangle.
An angle that is greater than a right angle is called an obtuse angle.
And a triangle with one obtuse angle is called an obtuse triangle.
Other kinds of triangles have special names that describe the way they
look.
A triangle that has two sides, or “legs,” that are equal in length, and
two equal angles at the base, is called an isosceles (eye
[sahs]{.smallcaps} uh leez) triangle. Isosceles means “equal legs.”
A triangle that has three sides that are equal in length, and three
equal angles, is called an equilateral (ee kwuh [lat]{.smallcaps} uhr
uhl) triangle. The word equilateral means “equal sides.”
A triangle that has sides of three different lengths, and three unequal
angles, is called a scalene (skay [leen)]{.smallcaps} triangle. The word
scalene means “uneven legs.”
