Learn about Classifying Triangles In this lesson, let’s learn how to classify triangles according to its sides and angles. So I'm going to basically tell you a little trick to follow. When it comes to sides of a triangle, if we have all three sides that are different in length then it’s called a scalene triangle. If two of the sides are equal then it’s called an isosceles triangle. If all three sides are equal, it’s called an equilateral triangle. So it’s either equilateral, isosceles or scalene, all three sides equal, two sides equal, one side is equal. Then as far as the angle goes, we just look for is one of the angles obtuse which greater than 90 degrees or is one of the angles right which is equals to 90 degrees or all three angles acute which means less than 90 degrees. So let’s use this to name these triangles. In this we can see this is an obtuse angle, so we know this is an obtuse triangle. One angle is obtuse. We don’t see any right angles. All three sides are different so this scalene obtuse triangle, simple. Second, let’s look at two sides are equal as marked by the marks. So these two sides are equal which means is an isosceles triangle and we can see one angle is 90 degrees so it’s a right angle triangle, isosceles, right triangle. First we look at the sides, two sides equal, isosceles, one angle right angle that’s right angle and then triangle. Let’s do the third one. The third one we can see it’s a right triangle, right so we know the angle is right. None of the sides are equal, so it’s a scalene right triangle. You get the idea. Fourth one, well we can see these two sides are equal so it’s an isosceles triangle. But this is an obtuse angle. So it’s an isosceles obtuse triangle. It doesn’t save 90 degrees, I can see that it’s greater than 90, 90 degrees would be until here. Let’s try the last one. Actually, this is the last one. So we’ve got four different triangles, scalene obtuse, isosceles right, scalene right and isosceles obtuse and the way we do that again to recap, we look at the sides if all three sides are different in length and scalene. Two sides are equal, isosceles, all three equal equilateral. If one of the angles is more than 90 degrees, it’s an obtuse triangle. If one of the angles is 90 degrees, it’s a right angle triangle and if all three are acute angles, then it’s an acute triangle.