How to Classify Triangles

In this lesson, let’s learn how we classify triangles. And we will identify each of the triangles given below as either equilateral isosceles, or scalene which are basically a measure of whether this sides are equal or not. And then identify each of them as acute, right obtuse which is a measure of their angles.

So every triangle is named depending on whether the sides are equal or not and whether the angles are what size are the angles. So here’s a simple rule to follow. If all three sides are equal, it’s an equilateral triangle. If two sides are equal, it’s an isosceles triangle. If no sides are equal, then it’s a scalene triangle. On the angle side, here’s the rule of thumb. If all three angles are equal, well that’s called an equilateral triangle anyway. But if one of the angles equals 90 degrees, then it’s a right triangle. If one of the angles is greater than 90degrees, then it is an obtuse triangle. If all angles are less than 90 degrees, then it’s an acute triangle. We don’t have to worry about all three angles being equal because all three angles being equal will give us all three angles being 60 degrees which is acute anyway.

So now let’s change the color of our pen and look at each one of this, what can we see? Well all three sides are different sizes. So immediately I can say this one is a scalene triangle. And what’s the size of the angles less than 90, less than 90, less than 90, so this is an acute scalene triangle. Here two sides are equal, two sides equal, isosceles. Angles all smaller than 90 degrees, so acute isosceles triangle. What do we see here all three sides are different, so it’s scalene all three sides different scalene. And we see one angle is 90 degrees so it is a right scalene triangle.

Let’s scroll on and do the second set. What do we see here? We see all three sides are equal the moment all the three sides are equal that’s an equilateral triangle. If the equilateral triangle all angles are 60 degrees, so it makes an acute anyway. Here we see two sides are equal right, so it’s an isosceles triangle. But we see that the angle is greater than 90 degrees, so this is an obtuse isosceles triangle.

And third one we see the angle as right 90 degrees, so it’s a right triangle and two sides are equal so that’s a nice isosceles triangle, right? Key thing to remember is we name a triangle depending on first whether the angles are one angle is right which is 90 degrees, it’s greater than 90 or all three angles less than 90. And the second part of the naming is whether the sides are all equal and which case it’s an equilateral to or equal isosceles and I’m not on the sides are the same scalene.