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### Course: Class 9 > Unit 13

Lesson 1: Coordinate geometry 3.1# Introduction to the coordinate plane

When we use coordinates to locate a point on a coordinate plane, we use two numbers to specify the horizontal (x) and vertical (y) positions. The first number in a coordinate pair tells us how far to move left or right from the origin, while the second number tells us how far to move up or down.

## Want to join the conversation?

- Why do you need to say the X axis first, and then the Y axis?(57 votes)
- Because if people changed it back and forth, people would get very different graphs(77 votes)

- why is there only a Y and X coordinate istead of random letters?(20 votes)
- On a 2 dimensional plane, the axes are named x and y, hence there are only x and y coordinates. On a 3D plane, you would also have a z coordinate. You can also look at it from an algebraic perspective where when you are comparing 2 equations, x and y are generally used as the two variables for graphing linear equations(36 votes)

- can there be coordinate planes in 3D (like X Y Z)?(20 votes)
- Yes, imagine being in a square room, one of the corners could be an origin, so the floor is the xy plane (where z=0), but you could locate any point in the room (such as making the units in feet) by being an x, y, and z distance from this origin. To get really fancy, the adjacent room or the hallway might be considered negative numbers depending on where they are related to the origin (a basement might be negative in the z direction).(29 votes)

- why does the x and y axis have to be in a spacifice spot?(12 votes)
- In math,the x-axis must be horizontal & the y-axis has to be vertical,or you probably would end up with a whole different situation.(21 votes)

- Why does the coordinate start with the x-axis and not the y-axis?(11 votes)
**This to avoid confusion**. If someone was to just give you two coordinates, you would have to a ask them which number is for which axis since flipping the coordinates can give you two very different graphs(17 votes)

- Why does it matter how far you go to the right or the left(5 votes)
- Good Question!

It matters because to plot the given coordinates on the cartesian plane you have to move along the x-axis horizontally in the left or right direction and the same for the y-axis in the vertical direction.

For example, if you have to plot (3,4) on the cartesian plane you have to move 3 points to the right (because 3 is positive) and 4 points in the upward direction (because 4 is also positive). The resulting exact point where these two coordinates meet is your correct answer.

Hope this Helps!(16 votes)

- y axis first or x axis first confused(2 votes)
- Just like in the alphabets x comes before y in the same way in coordinates system the x-axis comes first and then the y-axis.

So for example in (2,3) the x-axis coordinate is 2 and the y-axis coordinate is 3.

This is extra information - x-axis is also called the abscissa and the y-axis is also called the ordinate.(13 votes)

- why is there only a Y and X coordinate istead of random letters?(7 votes)
- This is because this is on a 2D Plane where only X and Y cords exist also because this is for starters in this subject(2 votes)

- Why do you need to put a comma when you put the coordinate down(0 votes)
- Doing this is called an
*Ordered Pair*. The comma separates the x value from the y value. Think about if they were connected. It would be a lot harder to tell them apart, don't you agree? Also, we use (,) rather than (:) because (:) represents a ratio rather than an ordered pair.

Hope this helps! -`Johnny Unidas`

(16 votes)

- when do we use geometry?(5 votes)
- We use geometry when building just about anything of if your an architect or even when making pizza, yes pizza. So in conclusion we use geometry every day during our normal lives.(1 vote)

## Video transcript

- [Instructor] You're probably familiar with the notion of a number line where we can take a number and associate it with a point on the number line. So for example, the number two, I would go, I would start at zero, I'd go one, two to the
right, and I would end up right over there. What we're going to do in this video is think about how do we take two numbers and associate them with a
point on a plane like this. So for example, you might
have the two numbers three, comma five. How can these two numbers be represented as a point on the plane, or how can these tell you
where a point on the plane is? So let's first get a
little bit of terminology out of the way. So what we have here,
this is often known as our coordinate plane. These two numbers you could
view as our coordinates. Let me write this down. These are coordinates. These black lines are known as the axes. Each one is an axis and the
one that goes left right here, this is known as the X axis, or it's typically known as the X axis. In the future it might
be called other things, and the one that goes up down
in the vertical direction, this is typically known as the Y axis. As you go further in math we
might call it other things, but most of the time it's
going to be called the Y axis. So how does three comma five, how do these coordinates
specify a point on this plane? Well the way that we
typically work it through, the standard way that people
will interpret these points, it'll say all right,
this first coordinate, this is our X coordinate. This is our X coordinate. You can view it as how far
do we move to the right along the X axis. So what you would do
is you'd say all right I'm gonna start right here where my axes intersect and I'm
gonna go three to the right. One, two, three. So my X coordinate says,
all right my point is going to be this far to the right. This far to the right. So it's gonna be somewhere
on this vertical line. This dotted line that I'm showing. Everything on this vertical line has an X coordinate of three. Now what's the Y coordinate? Another way I should say
it, the second number right over here. This is the Y coordinate. Y coordinate. This tells us how far do we move up. So one way to think about it, you could start back
where the axes intersect. This point is actually called the origin. Let me write that down. That is the origin, and so starting at the
origin, move five up. One, two, three, four, five. So everything on this
horizontal line that I'm drawing has a Y coordinate of five. So what point uniquely has
both an X coordinate of three and a Y coordinate of five? Well you can see where
those two lines intersect, right over here. Actually let me do that same blue color that I wrote the coordinates in. So this point right over here, it has an X coordinate of three, and it has a Y coordinate of five. That is the point three comma five. Now what is the coordinate of the origin? Well the origin is zero
to the right of the origin and it's also zero above the origin. So the coordinates there,
the X coordinate is zero, the Y coordinate is also zero. Let's do a few more examples. So let's say that I
wanted to plot the point two comma five. Why don't you pause this. Oh let me use a different number. Two comma four. Why don't you pause this video and think about where that point would be on this coordinate plane. All right let's do it together. So the first number is going
to be our X coordinate. It tells us how far do
we move to the right. So we move two to the right, and then our second number
says how far do we move up. So first we're gonna move two to the right and then we are going to move four up. So you can say one two to the right and then one two three
four right over there. This right over here is
the point two comma four. Notice is X coordinate. How far to the right of the origin it is. That is two and its Y coordinate, how far above the origin is. That is four. Now let's go the other way around. Let's say that I were to give you, if I were to give you this
point right over here. What would its coordinates be? Pause the video and
try to figure that out. All right, well we know
it's gonna be two numbers. So I'll do something comma something. Now the first something,
that's going to be our X coordinate. You could think of it, what point on the X axis are we above? You could think about how far to the right of the origin we are, and you can see that your X
coordinate right over here is if we just drop a vertical
line straight down from that point it hits the X axis at four. So that is four. Another way to think about
it we're one two three four to the right of the Y axis. Now how high are we? How high above the X axis are we? Well we're one above the X asis. So this is gonna be four comma one. Another way to think about
it, if you just take a line and you go straight to the left you're going to hit the Y axis
at the one right over here. So the coordinates here
are four comma one. Now just so we don't get confused, and when you first learn this, the main point of confusion is remembering that okay the first number
is the X coordinate, the second number is the Y coordinate. One comma four would be a different point. One comma four would be
okay one in the X direction and then four in the Y direction. So this is one comma four over here. So it's very important to
realize that the standard way of interpreting these numbers is that the first one
says how far do you move to the right of the origin, or how far do you move along the X axis or where are you on the X axis, and the second number
is how far do you move in the vertical direction, or where are you relative to the Y, or where are you in
the vertical direction?