Do you know your shapes? Sure, if you see a circle, you probably can identify it, right? But what about those shapes with multiple sides and lots of angles? Those can be a bit more difficult.

How do you go about identifying shapes? Many kids begin by counting the number of sides. For example, if you see a shape with four sides, what would it be? Depending upon the lengths of the sides, you’ve got either a square or a rectangle.

If you run across a shape with three sides, what do you call it? If you said triangle, you got it! But what kind of triangle is it? Did you know that there’s more than one type of triangle? It’s true! The simple three-sided triangle can be further defined in a number of ways.

In addition to three sides, every triangle also has three angles. Those three angles always add up to 180º. Mathematicians have given triangles special names based upon how many of those angles are equal.

For example, an equilateral triangle has three equal angles (and three equal sides). Each angle in an equilateral triangle has 60º, since three 60º angles would add up to 180º.

An isosceles triangle has two equal angles (and two equal sides). The angles can vary, as long as two are equal. For example, a triangle with two 70º angles and one 40º angle would be an isosceles triangle.

If you’ve been counting carefully, you know there’s one more option. If a triangle doesn’t have two or three equal angles, it could have zero equal angles. These triangles, which also don’t have any equal sides, are known as scalene triangles.

Can you believe that there are three different types of triangles? But wait…there’s more! Geometry experts weren’t satisfied with just three types of triangles. They also decided to classify triangles based upon the type of angles inside the triangle.

For example, some triangles have a right angle. That’s an angle equal to 90º. These are known as right triangles. Some triangles, however, have three angles that are all less than 90º. Such a triangle is called an acute triangle. A triangle can also have an angle more than 90º. Those triangles are called obtuse triangles.

Of course, there are even more varieties of triangles. You can combine the names based on their features. For example, a triangle with angles of 45º, 45º, and 90º is a right isosceles triangle. It’s a right triangle because it has a 90º angle. It’s an isosceles triangle because it has two equal angles of 45º.

So the next time you see a shape with three sides, don’t just shout “triangle!” Instead, try to figure out what kind of triangle it is.

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Standards:
*CCSS.MATH.CONTENT.1.GA.1, CCSS.MATH.CONTENT.2.GA.1, CCSS.MATH.CONTENT.4.GA.2, CCRA.L.3, CCRA.L.6, CCRA.R.1, CCRA.R.2, CCRA.R.10, CCRA.SL.1