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Geometry: High School15 chapters | 160 lessons

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Lesson Transcript

Instructor:
*Jeff Calareso*

Jeff teaches high school English, math and other subjects. He has a master's degree in writing and literature.

Line segments in triangles are just clutter; they have special names and qualities depending on how they're drawn. In this lesson, we'll learn about medians, altitudes and angle bisectors.

I don't claim to have much fashion expertise. Well, not when it comes to people. I don't know a clutch from a tote or why you'd need so many purses.

But when it comes to triangles, I'm much more confident. Your basic triangle looks like this.

It has three sides and three angles. It has corners that we call vertices, or a vertex for just one.

This triangle looks just fine. But it's lacking... something. In this lesson, we're going to learn about triangle fashion. We're going to add lines to our triangles that take them from 'meh' to 'wow.' These lines serve different roles in triangles, just like purses. Well, I think like purses. I'm still not sure about those.

First, let's draw a line from a vertex across a triangle like this.

We call this a median. A **median** is a line segment drawn from the vertex of a triangle to the midpoint of the opposite side.

It splits the opposite side into two equal line segments. We know it's a median if we have those equal line segments. It's like knowing someone paid attention while getting dressed if his socks match.

Depending on how formal our triangle's outing is, we can have up to three medians, one from each vertex. When we draw three medians, they always meet at a single point.

Medians are kind of like belts. They divide triangles in two. In fact, the two new triangles formed by adding a median have equal areas. And these six triangles formed by three medians also have equal areas.

So if medians are belts, what about earrings? Ok, this metaphor is venturing further into the unknown for me, but what happens with big earrings that are, um, dangly? They hang straight down, perpendicular to the ground, right? And why do they do that? Gravity.

And what's the triangle equivalent? Altitudes. An **altitude** is a perpendicular line segment drawn from a vertex of a triangle to the opposite side.

In our triangle here, if we draw a line from *A* perpendicular to the opposite side, it's an altitude. We could do this from any vertex, but we most commonly see it from the top.

Think of it like that earring. Gravity pulls it straight down. Unlike a median, an altitude doesn't necessarily split the opposite side into equal segments. In fact, it only will in two types of triangles.

In an equilateral triangle, all the angles are equal. Here, the altitude comes right down the middle and, in fact, is the same as the median.

This is also true in an isosceles triangle. Well, it's true for one of our altitudes. If we draw the other two, they clearly aren't also medians.

In a right triangle, two of the altitudes are actually sides of the triangle, since the sides already meet at right angles.

Here, we don't have matching socks, do we?

Also, an altitude isn't always inside a triangle. When there's an obtuse angle, we have an altitude that's actually outside the triangle. This is like an earring of someone leaning to the side for some reason. And instead of matching socks, it's like wearing a blue sock on one foot and hat on the other.

Let's consider one other type of triangle accessory. An **angle bisector** is a line segment drawn from a vertex that bisects, or divides in half, the vertex angle.

An angle bisector looks like this.

It forms two equal angles. That's its defining characteristic. So if an altitude is like a dangling earring, always hanging down due to gravity, an angle bisector creates a pair of matching earrings. And these aren't dangling earrings. They're the short kind. Do they have a name? Probably.

As with medians and altitudes, triangles can have three angle bisectors, and they always meet at a single point.

In an isosceles triangle, we have one angle bisector that is also a median and an altitude. Talk about versatility. In an equilateral triangle, all three lines are all three types. That's my style. Why buy different shoes for different occasions when you can just wear sneakers everywhere?

In summary, we learned about three different types of line segments related to triangles.

First, we learned about medians. These lines extend from a vertex and hit the midpoint of the opposite side, dividing it in half.

Then we learned about altitudes. These lines extend from a vertex and are perpendicular to the opposite side.

Finally, we learned about angle bisectors. These are line segments that bisect the angle of the vertex from which they're drawn.

When you complete this video lesson, you could have the capacity to:

- Identify median, altitude, and angle bisector lines in a triangle
- Describe their characteristics
- Compare isosceles and equilateral triangles

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Geometry: High School15 chapters | 160 lessons

- Area of Triangles and Rectangles 5:43
- Perimeter of Triangles and Rectangles 8:54
- How to Identify Similar Triangles 7:23
- Angles and Triangles: Practice Problems 7:43
- Triangles: Definition and Properties 4:30
- Classifying Triangles by Angles and Sides 5:44
- Interior and Exterior Angles of Triangles: Definition & Examples 5:25
- Constructing the Median of a Triangle 4:47
- Median, Altitude, and Angle Bisectors of a Triangle 4:50
- Properties of Concurrent Lines in a Triangle 6:17
- Go to High School Geometry: Properties of Triangles

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