Coordinate Geometry: Slope-Intercept Form

Now that we’ve looked at X and Y intercepts, it’s time to look at the slope-intercept form.

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Transcript: Slope-Intercept Form

Now that we’ve talked about slope and intercept, we can talk about the slope-intercept form of a line. I’ll start by saying the equation of any line can be stated in several different algebraically equivalent forms. We could have x and y on the same side of the equation, or we could solve for x or solve for y.

These algebraic changes don’t change the underlying mathematical information. No matter how we rearrange the algebra, it’s the same line that’s represented. So as long as we do the same thing for both sides of the equation, we’re not actually changing the mathematical information in the equation. Of all the possible algebraic representations, mathematicians have selected one as particularly useful.

If we solve the equation of a line for y, getting y on one side by itself equal to everything else–then we automatically put the equation into what is called slope-intercept form. Now slope-intercept form, you may know this by the name y = mx + b. y = mx + b is another way of saying slope-intercept form. In this equation, y = mx + b, m is the slope of the line, and b is the y-intercept of the line.

And that’s why we call this slope-intercept form, because two of the numbers in the equation are the slope and the intercept. If we simply solve the algebra for y, we automatically get these two very important quantities. The slope and the y-intercept. We can see that m must be the slope because whenever x increases by 1, y has to increase by m.

We can see that b must be the y-intercept, because if x = 0, then y = b. It’s easy to put any equation into slope-intercept form. And once it’s in that form, it’s easy to understand where the graph will go. Starting from the y-intercept, we can take the slope steps that we discussed in the Slope lesson. 1 unit to the right, m units up, 1 unit to the left, m units down, etc.

Practice Problem: Slope-Intercept Form

Here’s a practice question, pause the video and then we’ll talk about this.

Okay, find all the points (a, b) on the line y equals negative, four-thirds x plus 2, such that a and b are both integers with absolute values less than or equal to 10. Okay, well certainly one point that we know is on the line is the y-intercept. We could see the y-intercept is 2, so we’ll just write this as in point point form (0,2), that’s the y-intercept.

Now that slope tells us, that if we start moving left, we can move over with a rise of -4. In other words a drop of 4 in a round of 3. So that’s over 3, 3 to the right, down 4. And so we go 3 to the right. That puts us at x equals 3, and down 4, that puts us at y equals -2.

We can go 3 to the right and down 4 again. That will put us at (6, -6), and then 3 to the right down 4, bring us to (9, -10). We have to stop there because we’ve reached an absolute value as large as 10. Now we can go the other way. Again start at the y-intercept.

Now we’re gonna go to the left threes. In other words, the x intercept is gonna go down by 3. The x coordinate, and the y is gonna go up by 4. So now we’re gonna go backwards, go left to -3, and the y-intercept is gonna go up by 4, so that’s (-3,6). And then take another step to the left, that would bring the x down to -6, and the y would go up to 10.

Again, we have to stop there. And so those are the points, one, two, three, four, five, six, those six points are the points that satisfy this condition. We can see these six points on this line.

Test Questions

Here’s another practice question, pause the video and then we’ll talk about this, Okay? Let me point out that this question is a bit easy. This is really too easy to be a test question. But the important thing to appreciate is, the skill here is something that could very well be part of a test question.

In other words, you might have to get the equation, find the slope, and then do something else with the slope. That would be what the test would ask you to do. But certainly one absolutely non-negotiable skill you need to have is given an equation, you need to be able to find the slope.

slope-intercept form, slope - magoosh

Image by sirtravelalot

And of course what we’re gonna do is we’re just gonna solve for y, and put this into slope intercept form. So there’s the equation. We will subtract 3x on both sides, so that moves to the other side. Now we’re gonna divide both sides by 5, and we get y = -3/5x + 8/5.

So, we can see that the slope is negative three fifths. We can also see that the y-intercept would be positive eight fifths. We get that information kid of as a bonus. But we know that the slope is -3/5. Notice that horizontal lines have a slope of zero, because they are all run with no rise.

If a horizontal line has a y-intercept of 4, then m = 0 and b = 4. If we put it into slope-intercept form it would be y = 0 times x + 4. Of course that just becomes y = 4, which is the standard form of a horizontal line. So notice that the default form for a horizontal line can be interpreted as a kind of slope intercept form.

We’re getting y equals the y intercept because the slope is 0.

Vertical Lines: Undefined

Vertical lines are very different. First of all, vertical lines have what is known as an undefined slope, because the slope fraction is always something divided by zero. And of course, as we know, you’re not allowed to divide by zero. If we divide by zero, we leave the world in which mathematics makes any sense whatsoever.

And so, whatever the slope is, it’s something that does not make any sense mathematically. So the word we use for that is undefined. Not defined by anything within mathematics. That’s the slope of a vertical line. Vertical lines run parallel to the y-axis and never intersect it, except of course the only vertical line that intersects the y-axis is of course the y-axis itself.

But other than that, no vertical line even has a y-intercept, because they run parallel to the y-axis. Therefore, the standard form for vertical lines, x = K. In other words we’re specifying the x-intercept, and that specifies the entire line that’s completely unrelated to slope-intercept form. So, vertical lines are very different. But the standard form of horizontal lines is really one special case of slope-intercept form.

Summary of Slope-Intercept Form

In summary, if we solve the equation of any line for y, we automatically put the equation into slope intercept form. In other words, y = mx + b, where m is the slope of the line, and b is the y intercept. This form makes it easy to graph the line, and to understand where it goes.

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