 Hi! Welcome to Math Antics. We're continuing our series on Geometry  
and today we're going to learn about angles.
In our last video, we learned about points and lines,
and that's good because 
we are going to need lines to make angles.
So let's start with a couple of lines 
that are in the same plane.
We're only going to be dealing with 
 two-dimensional geometry in this video. 
These lines are conveniently called 
 Line AB and Line CD.
Now the important thing to notice about these two lines is that they're pointing in exactly the same direction. 
So, even if we extended  them forever, they would never cross or even get closer together.
When two lines are arranged like this, 
we call them 'parallel'.
You've probably heard the term 'parallel' before… 
like parallel parking, or a parallel universe,
or parallel bars.
Okay, so parallel lines are lines that will
never cross, even if they go on forever...
but what if I take one of our lines and 
give it a little nudge?
Now the lines aren't parallel anymore. 
In fact, they cross at this point right here.
Let's name it Point P. 
When lines cross at a point like this, 
we say that they intersect,
and we call the point an 'intersection'.
And when lines intersect, they form 'angles'. You can think of the angles as the spaces, or shapes,
that are formed between the intersecting lines.
 These intersecting lines form four angles: 1, 2, 3, 4.
But instead of calling them angle 1, 2, 3 and 4,
 in Geometry, 
we name them by the points used to make them.
For example, this angle here can be called Angle DPB 
because if you trace along those points 
(like connect the dots) they outline that angle.
And this angle here... we can call that Angle APD,
because connecting those dots forms angle.
Now when naming angles, 
there's a nice shorthand we can use.
 Instead of writing the word 'angle' over and over again,
we can just use the angle symbol instead, 
which looks like this.
But there's an even simpler way to name angles.
To learn that way, 
let's erase all the points and letters on our lines
except for the intersection point and this one point here.
Now let's imagine that the line-segment between these two points can rotate around the point of intersection,
just like a clock hand rotates 
around the center of a clock. 
Let's also imagine that as we rotate the line segment,
the point out at the end leaves a trail, 
like if a pencil was attached to it.
The trail (or path) that is left when we rotate the line-segment all the way around forms a circle.
But if we only go part way around,
then it forms part of a circle that we call an 'arc'.
This arc can represent the angle
that is formed when we rotate the segment
from one position to another, 
like from this line to that line.
And now, if we shrink down that arc so that
it's close to the intersection point,
and then put a letter by it, like the letter 'A',
we have another way of showing an angle...
Angle A.
And we can do this with any angle,
so the angle up here...
we can also draw an arc and call it Angle B.
So whenever you see a letter next to a little arc like this,
it means that it's the name of the angle 
formed by that arc.
Alright then, so now we have a diagram that shows Angle A and Angle B,
and you might notice that those angles 
aren't the same size. 
B seems to be bigger than A.
But what if we rotate one of our lines until
the angles do look like they're the same size?
Now our angles look kind of like a plus sign.
Lines arranged like this are called 'perpendicular'.
Perpendicular lines are lines that form square corners when they intersect.
And these square corner angles have a special name in Geometry because they are really important.
We call them 'right angles'.
There is even a special symbol that we use to show when an angle is a right angle. 
Because they form square corners, 
we use a little square instead of 
the arc that we use for the other angles.
So whenever you see this symbol, 
you know the angle you are looking at is a right angle,
and that the lines that form it are perpendicular.
Okay, now that you know what a right angle is, let's look at a simple one that's made from just two rays.
What will happen if we take the ray pointing up and rotate it like the hand of a clock
a little to the right… a little bit clockwise?
Well, we don't have a right angle any more
because the rays are no longer perpendicular.
Instead, we have an angle that is smaller
(or less) than a right angle.
Angles that are less than right angles 
are called 'acute angles'.
On the other hand, if we rotated our ray to
the left instead of the right,
we would get an angle that's bigger 
or greater than a right angle. 
Angles that are greater than right angles 
are called 'obtuse angles'.
So, there are three main kinds of angles 
that you need to know about. 
Right Angles, acute angles and obtuse angles.
Well actually, there's one more type of 
angle that's pretty important,
but it's kind of a strange one. 
It's called a 'straight angle'.
A straight angle is just what we get when we rotate our rays so that they point in exactly opposite directions.
The result looks just like a straight line, 
which is why it's called a straight angle.
Alright then, there's just a few more important geometry terms that we need to learn in this video.
 Let's look at our simple right angle
again that's made from two rays.
But this time, let's draw a third ray that
cuts that right angle into two smaller parts.
Now, because the angle that we divided up
was a right angle, 
we know that the two new smaller angles 
combine to form a right angle.
And in geometry, any two angles that to form a right angle are called 'complementary angles'.
And we can do the same thing with a straight angle.
 If we take a straight angle (made from two rays) 
and divide it with a third ray,
two new smaller angles are formed.
And those two angles combine to form a straight angle.
We call these angles 'supplementary angles'.
So, complementary angles 
combine to form a right angle, 
and supplementary angles 
combine to form a straight angle.
Alright, that's all were going to learn about angles 
in this video.
And if you are new to Geometry, 
it might seem like a lot,
so let's do a quick review of 
all the new geometry words we've learned.
Lines that point in exactly the same direction will never cross and are called 'parallel' lines.
When lines do cross, they cross at a point
called an 'intersection'.
Lines that intersect form 'angles'. You can
think of angles as the spaces between the lines.
Angles can be named by the points that
form them... just like connect the dots.
An 'arc' is a part of a circle. Arcs can be used to represent an angle between two intersecting lines.
When intersecting lines form all exactly equal angles, the lines are 'perpendicular'.
Perpendicular lines form 'right angles'. 
Right angles are square corners, 
and we use a special square symbol 
to show that an angle is a right angle.
An angle that's smaller, or less than a right
angle is called an 'acute angle'.
An angle that's bigger, or greater than a
right angle is called an 'obtuse angle'.
A 'straight angle' is formed by two rays pointing
in exactly opposite directions. 
A straight angle is really just a straight line.
Two angles that combine to form a right angle
are called 'complementary angles'.
Two angles that combine to form a straight
angle are called 'supplementary angles'.
In our next geometry video, we're going to
learn more about angles and how to measure them.
Thanks for watching Math Antics, and I'll
see you next time!
Learn more at www.mathantics.com
