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Current time:0:00Total duration:6:49

CCSS Math: 4.MD.C.5

- Let's say we have one ray
over here that starts at point A and then goes through point B, and so we could call this ray-- let me draw that a little bit straighter, we could call this ray AB, ray AB starts at A or has a vertex at A and let's say that there's also a ray AC, so let's say that C is
sitting right over there and then I can draw another
ray, goes through C, so this is ray AC. What's interesting about these two rays is that they have the exact same vertex, they have the exact same vertex at A. In general, what we have
when we have two rays that have the exact same
vertex, you have an angle and you're probably already
reasonably familiar with the concept of an angle, which I believe comes from
the Latin for "corner", which makes sense, this looks
like a little bit of a corner right over here that we see at point A and the geometric definition, or the one that you're most likely to see, is when two rays share a common vertex and that common vertex is actually called the vertex of the angle, so A is vertex. Not only is it the vertex
of each of these rays, ray AB and ray AC, it is also the vertex of the angle. The next thing I want to think
about is how do we label? How do we label an angle? You might be tempted to
just label it angle A but I'll show you in a second
why that's not going to be so clear to someone based on where our angle is actually sitting. The way that you specify an angle, and hopefully this will
make sense in a second, is that you say "angle", this is the symbol for angle and it actually looks
strangely similar to this angle right over here, but
this little pointy thing or it almost looks like a less-than sign but it's not quite, it's flat
on the bottom right over here. This is the symbol for angle, you'd say angle BAC or you could say angle CAB. In either case, they're kind of specifying this corner, or sometimes
you could view it as this opening right over here. The important thing to
realize is that you have the vertex in the middle of the letters. You might be saying, "Wait,
why go through the trouble "of listing all three of these letters? "Why can't I just call this angle A?" To see that, let me show
you another diagram. Although the geometric
definition of an angle involves two rays that
have the same vertex, in practice, you're
going to see many angles that are made up of
lines and line segments. You could imagine that you could continue those line segments on
and on in one direction and then they would become rays so, in that way, they're
consistent this definition. Let's add one line segment
that looks like that, let me label some points here, so we've already used ABC
so let me call this D and E, points D and E, so this
is line segment DE. Let's say I also have line segment FG, FG, and let's say this point where these two line segments intersect, let's called that point, point H. How could we specify this
angle right over here? Can we just that angle H? Well, no, because if we just said angle H, the angle that has a vertex H, it could be this angle right over here or it could be this angle right over here, let me draw it this way, you could view it that way or it could be that angle over there, it could be this angle over here, it could be this angle over here or it could be that angle over there and so the only way to really specify which angle you're talking about well is to give three letters. If you really did want
to talk about that angle right over there, you
would call that angle EHG, so that is angle EHG or you could actually specify that or you could call that angle GHE. GHE If you wanted to specify
this angle right over here, if you wanted to specify
that angle right over here, the one made up of, if
you imagine that ray and that ray, if you were to keep on going past those points, then you
could call that angle DHG angle DHG, or angle GHD, or angle GHD, I think you get the point. This angle up here could be FHE or EHF and this one could be FHD or DHF and when you do it this
way, it's very clear what angle you are referring to. Now that we have a general
idea of what an angle is and how do we denote it with symbols, the next thing you might
be curious about is, it doesn't look like
all angles are the same, it seems like some angles open up or are more open than others and some are a little bit
more closed in than others and that actually is the case. For example, let's take two angles here, let's say I have one angle
that looks like that, so I'll start reusing letters, so let's say that this is A, B and C, I can make these rays,
I could keep on going and make them rays if I like or I could just keep
them as line segments, so right over here I have angle BAC and let's say over here, I have angle-- so let me draw another one, and let's say this is angle XYZ and, once again, I could
draw them as rays if I like, to go on and on and on, so it's angle X, Y and Z. When you just look at these, you just eyeball these two angles, it looks like this one is more open, so this one looks more open while this one over
here looks more closed, so it looks like it's more closed, at least relative to
this one, more closed. Maybe when we measure angles, we should measure it based on
how open or closed they are and that actually is the case, and so without even telling
you how we measure an angle, you could say that the
measure of angle XYZ, the measure of this angle is greater than the measure
of this angle right over here and any convention we
use for measuring angles is essentially going to be a measure of how open or how closed
an angle actually is and I'll take that up in the next video where we'll see how to
actually measure an angle.