Coordinate Geometry: Graphing Lines

What do you know about graphing lines? Do you want to know more? Check out our video to learn all you need to know on this subject!

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Transcript: Graphing Lines

So, now we’ll talk about graphing lines. You may well remember from high school that a big topic of the x-y plane has to do with graphing lines and finding equations of lines. So for example, we might have a line like this, we might have to find the equation of a line. Or we might be given an equation, have to produce this line, something along those lines.

In this lesson, we will start with the very basics. As in many ways it’ll just be kind of a conceptual introduction to the idea of graphing lines on the plane. First, we have to focus on a few big ideas before we can actually get to the mechanics of how to graph a particular line.

Graphing Lines: Big Idea #1

So BIG IDEA #1, every possible line in the x-y plane has its own unique equation.

So you can say there’s a one to one pairing between a unique line and a unique equation. That’s a big idea, every line has its own equation.

Big Idea #2

BIG IDEA #2, for any given line all the points on the line have x- and y- coordinates that satisfy the equation of the line. So that’s a really big idea. That’s the deep idea and people don’t appreciate how deep that idea is.

On any line, there’s an infinite number of points–all infinity of those points, every single one of them. We could pick out any point at all on that line, find it’s x coordinate, y coordinate, and plug it in. It would satisfy the equation of the line. That is absolutely huge.

Big Idea #3

And finally, BIG IDEA #3, any linear equation that relates x to the first power or the y to the first power.

As long as there’s no multiplication or division of variables or something odd like square roots. As long as it’s just an ordinary x, an ordinary y, and a bunch of numbers that must be the equation of some line in the x-y plane.

Linear Equations

So for example we look at this, y is to the first power, x is to the first power. That has to be the equation of some line in the x-y plane, and that’s exactly why these are called linear equations.

graphing lines, linear equations - magoosh

Image by hafakot

You may remember back in Algebra, we were referring to these as linear equations. We were referring to them because every single one of them corresponds to a unique line in the x-y plane, those circuit base particular equations. Suppose the problem gave us that equation. We could find values that satisfy that equation and these would be points of the line. So for example, we could just plug in, if we plug in x = 0, then we’d see that we get 3y = 12. So y would equal 4.

That means that (0, 4) has to be one point on the plane. Similarly, we got plug in y=0. If y=0 then we get -4 = 12. We divide, we get x = -3, so that must be another point on the plane, x equals -3y=0. So we have two points, technically two points are enough to determine a line.

For example, if we plot those two points and just draw a straight line between them we get a graph of the line. We were able to draw the line simply by plugging in points. But this is not the most efficient way to plot a line. In the coming lessons of this module, we’ll learn much more efficient ways to plot the line that represents a particular equation.

So we’ll get to that, but first we have to make sure that we understand all the basics here. For now once again, the really big idea is that any point on the line has to satisfy the equation of the line. This idea can be tested in a variety of contexts.

Graphing Lines: Practice Problem

So here’s a relatively easy practice problem. Pause the video and then we’ll talk about this.

Okay, we’re given the equation of the line, and the equation of the line has this variable in it, the variable K. So we don’t know what K is but we’re told that the line must pass through the point (2, 1). Well we know that that point has to satisfy the equation of the line. So if we plug in X = 2 and y = 1, we’re gonna have to get an equation that works.

So we’ll do that. We’ll plug in x = 2 y = 1, what we get is 2k + 3k = 5k and 5k = 17. And K must equal 17/5, so that’s the value of K.

In summary, every line in the x-y plane has its own unique equation. Every point on the line satisfies the equation of the line.

And we can figure out the graph of a line by plotting individual points. This is one option, and we will learn other options in the later videos of this module.

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