Geometry: Triangles – I

In Geometry you need to learn a lot about triangles! Take a look at our first video on the subject for an introduction. Then follow up with the next few videos, all discussing different aspects of triangles!

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Transcript of Video

A triangle is a shape formed by three line segment sides. And of course triangles are important, first of all, there’s a lot of important facts about triangles themselves. And also we can understand many other shapes by thinking of them as triangles put together.

So they play a very important role in geometry. Let’s talk about the basic terms for triangles. The line segments are called sides obviously. Each corner where the two line segments meet when there is an angle, we call that a vertex and the plural of that world is vertices. So a triangle has three vertices.

All Triangles

In fact, every triangle has exactly three sides and exactly three vertices. This video lesson concerns some of the basic facts that are true for every single triangle on Earth. In later videos, we’ll start talking about some special triangles.

But this video’s really about all triangles. The most important and most remarkable is that for any triangle–no matter what the individual angles are–the sum of the three angles inside any triangle on earth is exactly 180 degrees.

That is an absolute remarkable fact proved first by Mr. Euclid. And it is 100% true for all triangles. So, why is this true? Why would that remarkable fact be true? Well, think about it this way. Consider the following diagram.

Triangle ABC could be any triangle. Line DE is drawn so that it is parallel to AC. So remember, we talked about in the last video when we had parallel lines and a transversal cutting across those parallel lines. Well, if we consider AB a transversal, then those two red angles BAC and ABE, those two red angles are equal.

Those are congruent angles there. If we consider BC a transversal then the two blue angles, ACB and CBD, those are also congruent. And of course, whatever that yellow angle is in the middle, we can see that if we look along the line, those three angles add up to 180 degrees. Because they lie along the line, red, yellow, blue, those three angles.

And so it must mean that the three angles in the triangle also add up, because it’s the same angles, they also add up to 180 degrees. So that’s why this works for any triangle. Understanding this may help you remember that fact and understand that a little bit more deeply. Now, some helpful terms for angles.

triangles, right angle - magoosh

Image by AliDesign


First of all, an acute angle is angle less than 90 degrees. A right angle of course is 90 degrees. And an obtuse angle is more than 90 degrees. We’ll be using these terms throughout our discussion of geometry and it’s possible that they could appear on the test. Because the sum of the angles in the triangle is always 180 degrees.

A triangle cannot possibly have two right angles or two obtuse angles. At least two of the angles in any triangle have to be acute. A triangle could have one obtuse angle and two acute angles, so we have a diagram here. And incidentally all the diagrams in these lesson videos are drawn to scale, so that is a scale diagram of a triangle with 20,120 and 40 degrees.

A triangle could have one right angle and two acute angles. Here’s a 30, 60, 90 triangle. Or a triangle could have three acute angles, so 50, 60 and 70 degrees. In a triangle the angles that any vertex is between two sides, it touches those two sides. So you could say it’s adjacent to those two sides.

For example angle ABC, the angle right here touches side AB and side BC. It’s adjacent to those two sides. The third side, the side that the angle does not touch, is opposite the angle. For example, that same angle ABC and side AC are opposite one another. So that’s an important relationship and we will be talking about that a lot.

Largest Angle Rule

The relationship of the angle and it’s opposite side. Now we can talk about another rule true for all triangles. The largest angle is always opposite the longest side, 100% of the time. The largest angle is opposite of the longest side and the smallest angle is always opposite the shortest side. In the triangle above across triangle scale, AB is clearly the longest sides.

So that means that angle C has to be the largest angle, and in fact, angle C looks like it’s obtuse. An obtuse angle and a triangle is always gonna be the largest angle. BC is clearly the shortest side, and so that means that angle A must be the smallest angle. So we absolutely know that this is always true.

Triangle Inequality Theorem

The final rule that is true for all triangles is called the triangle inequality theorem. The Triangle Inequality Theorem states that in any triangle, the sum of any two sides must be greater than the third side. Notice that this is a strict inequality. It’s not greater than or equal to, but strictly greater than.

Why is this true? Well, think about the triangle. Here’s a typical triangle. Clearly, the shortest distance between two points ha to be straight line path. That’s a fundamental geometric fact, the shortest distance between two points is always a straight line.

Well, that means the shortest possible distance between point A and point C must be the segment AC because AC is a straight path between A and C. The detour path from A to B to C has to be longer than AC because AC by definition is the shortest possible path. Therefore, it must be true that that detour path, AB + BC has to be longer than AC.

And we can say the same analogous thing for the other pairs of sides, that AC + BC has to be bigger than AB by itself. And AB plus AC has to be bigger than BC by itself. Another way to think about the triangle inequality, suppose the sum of two sides was less than the third side. What if we tried to construct say a triangle with lengths 2, 3, and 10?

So here’s a scale diagram, and you can kinda see there’s a problem here. Because no matter how we move that side of a 2 and a side of 3, we can rotate however we want, they’re not gonna touch. No matter where we move the short arms they are simply not gonna reach across that gap to connect. So we’re not actually gonna be able to join them up and close off the shape as a triangle.

No Open Triangles

So this does not form a legitimate triangle. It has to be closed. The three line segments all have to meet and enclose an area in order for it to be a triangle. So this will not work as a triangle. And this is why it’s impossible to have two sides in a triangle that sum to less than the third side.

What if the sum of the two sides equals the third side? Remember, the equals is not allowed by the triangle inequality either, it’s a strict greater than. So what’s the problem with equals? Well, what happens if we try and construct a triangle with sides 4, 6, and 10? So four plus six equals ten, what’s going on here?

Well you see the problem here, I mean yes, we can get the side of 6 and the side of 4 to connect up with each other, but the problem is, everything is flat. And what we have, there’s not really a geometric word for it, so I’m gonna call it a flat thing. That flat thing is not a triangle. A triangle has to have an inside, it has to have area.

So this flat thing does not meet our requirement for what a triangle is. This is what happens when we have two sides equal. Their sum is equal to the third side. And so this is why not only the less than case is excluded but also the equal to case is excluded. The sum of two sides has to be greater and only greater than the third side.

Suppose we know that two sides of a triangle are 8 and 13 and we wanna know the possible lengths of the third side. Well, certainly it’s true that if we take the sum of the unknown side plus 8 it has to be bigger than 13. Well, from this inequality, we could subtract 8 from both sides, and it means that that x has to be greater than 5.

So that’s one of the statements we have, and notice that 5 is the difference of 8 and 13. It’s also true that the sum of 8 and 13 must be greater than that third side. So it means that x has to be less than 21. Notice that’s the sum of the two sides, so we can say that x must be greater than 5 and less than 21.

That’s the allowable range for x, and in particular, notice that it is greater than the difference of the sides and less than the sum of the sides. Or we can generalize this. If we know that the lengths of two of the three sides of a triangle we know that there are P and Q,then the third side has to be greater than the difference and less than the sum.

Another Triangle Rule

Any side of a triangle must be more than the difference of the other two sides and less than the sum of the other two sides. So this is an alternate way to state the triangle inequality. In summary, the sum of three angles in any triangle must be 180 degrees. That’s the really big one, true for all triangles. Two angles of a triangle must be acute, the third one could be acute, right or obtuse, but it’s true for any triangle that at least two of the angles are acute.

Another property true for all triangles, the biggest angle is always opposite the largest side. And the smallest angle is always opposite the shortest side, true for all triangles. The sum of any two sides of a triangle is greater than the third side, true for all triangles. And another way to say that is any side of a triangle must be greater than the difference of the other two sides and less than the sum of the other two sides.

Everything on this summary is true for every triangle on Earth.

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2 Responses to Geometry: Triangles – I

  1. Garima Thakur March 16, 2020 at 10:23 am #

    Thank you Magoosh! School just got cancelled for 3 weeks and I’ve been so bored, but then I found this website and my brain got freed of boredom. I’m so happy to have a website that feeds my thirst for knowledge!

    • Magoosh Test Prep Expert
      Magoosh Test Prep Expert March 20, 2020 at 1:12 pm #

      Thanks for the kind words, Garima! I’m glad it was helpful. 🙂


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