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Triangles are usually classified by their side length and their angles.
If all three sides are different lengths, the triangle is called scalene. If two sides
are the same, it’s called isosceles. That’s Greek for “identical legs”. And if all
three sides are the same, it’s called equilateral. So a regular triangle is called an equilateral
triangle. Technically, an equilateral triangle is also an isosceles triangle, because it
does have two equal sides.
Notice the little marks we draw on the sides of the triangles, to indicate which sides
are the same length. Every side with the same number of marks is the same length.
Triangles are also classified by their angles. If all three angles are acute, then the triangle
is acute. If one of the angles is a right angle, then the triangle is called a right
triangle. And if one of the angles is obtuse, then the triangle is called obtuse.
So an equilateral triangle is also an acute triangle, because every angle will be the
same, sixty degrees. Isosceles and scalene triangles can be acute, right or obtuse.
Notice that it’s impossible to have more than one right angle or obtuse angle in a
triangle. Can you see why not?