Video Transcript: Writing Slope-Intercept Equation
Slope-intercept equation from graph So you may or may not already know that any linear equation can be written in the form y is equal to mx plus b. Where m is the slope of the line. The same slope that we've been dealing with the last few videos. The rise over run of the line. Or the inclination of the line. And b is the y-intercept. I think it's pretty easy to verify that b is a y-intercept. The way you verify that is you substitute x is equal to 0. If you get x is equal to 0-- remember x is equal to 0, that means that's where we're going to intercept at the y-axis. If x is equal to 0, this equation becomes y is equal to m times 0 plus b. m times 0 is just going to be 0. I don't care what m is. So then y is going to be equal to b. So the point 0, b is going to be on that line. The line will intercept the y-axis at the point y is equal to b. We'll see that with actual numbers in the next few videos. Just to verify for you that m is really the slope, let's just try some numbers out. We know the point 0, b is on the line. What happens when x is equal to 1? You get y is equal to m times 1. Or it's equal to m plus b. So we also know that the point 1, m plus b is also on the line. Right? This is just the y value. So what's the slope between that point and that point? Let's take this as the end point, so you have m plus b, our change in y, m plus b minus b over our change in x, over 1 minus 0. This is our change in y over change in x. We're using two points. That's our end point. That's our starting point. So if you simplify this, b minus b is 0. 1 minus 0 is 1. So you get m/1, or you get it's equal to m. So hopefully you're satisfied and hopefully I didn't confuse you by stating it in the abstract with all of these variables here. But this is definitely going to be the slope and this is definitely going to be the y-intercept. Now given that, what I want to do in this exercise is look at these graphs and then use the already drawn graphs to figure out the equation. So we're going to look at these, figure out the slopes, figure out the y-intercepts and then know the equation. So let's do this line A first. So what is A's slope? Let's start at some arbitrary point. Let's start right over there. We want to get even numbers. If we run one, two, three. So if delta x is equal to 3. Right? One, two, three. Our delta y-- and I'm just doing it because I want to hit an even number here-- our delta y is equal to-- we go down by 2-- it's equal to negative 2. So for A, change in y for change in x. When our change in x is 3, our change in y is negative 2. So our slope is negative 2/3. When we go over by 3, we're going to go down by 2. Or if we go over by 1, we're going to go down by 2/3. You can't exactly see it there, but you definitely see it when you go over by 3. So that's our slope. We've essentially done half of that problem. Now we have to figure out the y-intercept. So that right there is our m. Now what is our b? Our y-intercept. Well where does this intersect the y-axis? Well we already said the slope is 2/3. So this is the point y is equal to 2. When we go over by 1 to the right, we would have gone down by 2/3. So this right here must be the point 1 1/3. Or another way to say it, we could say it's 4/3. That's the point y is equal to 4/3. Right there. A little bit more than 1. About 1 1/3. So we could say b is equal to 4/3. So we'll know that the equation is y is equal to m, negative 2/3, x plus b, plus 4/3. That's equation A. Let's do equation B. Hopefully we won't have to deal with as many fractions here. Equation B. Let's figure out its slope first. Let's start at some reasonable point. We could start at that point. Let me do it right here. B. Equation B. When our delta x is equal to-- let me write it this way, delta x. So our delta x could be 1. When we move over 1 to the right, what happens to our delta y? We go up by 3. delta x. delta y. Our change in y is 3. So delta y over delta x, When we go to the right, our change in x is 1. Our change in y is positive 3. So our slope is equal to 3. What is our y-intercept? Well, when x is equal to 0, y is equal to 1. So b is equal to 1. So this was a lot easier. Here the equation is y is equal to 3x plus 1. Let's do that last line there. Line C Let's do the y-intercept first. You see immediately the y-intercept-- when x is equal to 0, y is negative 2. So b is equal to negative 2. And then what is the slope? m is equal to change in y over change in x. Let's start at that y-intercept. If we go over to the right by one, two, three, four. So our change in x is equal to 4. What is our change in y? Our change in y is positive 2. So change in y is 2 when change in x is 4. So the slope is equal to 1/2, 2/4. So the equation here is y is equal to 1/2 x, that's our slope, minus 2. And we're done. Now let's go the other way. Let's look at some equations of lines knowing that this is the slope and this is the y-intercept-- that's the m, that's the b-- and actually graph them. Let's do this first line. I already started circling it in orange. The y-intercept is 5. When x is equal to 0, y is equal to 5. You can verify that on the equation. So when x is equal to 0, y is equal to one, two, three, four, five. That's the y-intercept and the slope is 2. That means when I move 1 in the x-direction, I move up 2 in the y-direction. If I move 1 in the x-direction, I move up 2 in the y-direction. If I move 1 in the x-direction, I move up 2 in the y direction. If I move back 1 in the x-direction, I move down 2 in the y-direction. If I move back 1 in the x-direction, I move down 2 in the y-direction. I keep doing that. So this line is going to look-- I can't draw lines too neatly, but this is going to be my best shot. It's going to look something like that. It'll just keep going on, on and on and on. So that's our first line. I can just keep going down like that. Let's do this second line. y is equal to negative 0.2x plus 7. Let me write that. y is equal to negative 0.2x plus 7. It's always easier to think in fractions. So 0.2 is the same thing as 1/5. We could write y is equal to negative 1/5 x plus 7. We know it's y-intercept at 7. So it's one, two, three, four, five, six. That's our y-intercept when x is equal to 0. This tells us that for every 5 we move to the right, we move down 1. We can view this as negative 1/5. The delta y over delta x is equal to negative 1/5. For every 5 we move to the right, we move down 1. So every 5. One, two, three, four, five. We moved 5 to the right. That means we must move down 1. We move 5 to the right. One, two, three, four, five. We must move down 1. If you go backwards, if you move 5 backwards-- instead of this, if you view this as 1 over negative 5. These are obviously equivalent numbers. If you go back 5-- that's negative 5. One, two, three, four, five. Then you move up 1. If you go back 5-- one, two, three, four, five-- you move up 1. So the line is going to look like this. I have to just connect the dots. I think you get the idea. I just have to connect those dots. I could've drawn it a little bit straighter. Now let's do this one, y is equal to negative x. Where's the b term? I don't see any b term. You remember we're saying y is equal to mx plus b. Where is the b? Well, the b is 0. You could view this as plus 0. Here is b is 0. When x is 0, y is 0. That's our y-intercept, right there at the origin. And then the slope-- once again you see a negative sign. You could view that as negative 1x plus 0. So slope is negative 1. When you move to the right by 1, when change in x is 1, change in y is negative 1. When you move up by 1 in x, you go down by 1 in y. Or if you go down by 1 in x, you're going to go up by 1 in y. x and y are going to have opposite signs. They go in opposite directions. So the line is going to look like that. You could almost imagine it's splitting the second and fourth quadrants. Now I'll do one more. Let's do this last one right here. y is equal to 3.75. Now you're saying, gee, we're looking for y is equal to mx plus b. Where is this x term? It's completely gone. Well the reality here is, this could be rewritten as y is equal to 0x plus 3.75. Now it makes sense. The slope is 0. No matter how much we change our x, y does not change. Delta y over delta x is equal to 0. I don't care how much you change your x. Our y-intercept is 3.75. So 1, 2, 3.75 is right around there. You want to get close. 3 3/4. As I change x, y will not change. y is always going to be 3.75. It's just going to be a horizontal line at y is equal to 3.75. Anyway, hopefully you found this useful. Slope-intercept equation from slope & point A line has a slope of negative 3/4 and goes through the point 0 comma 8. What is the equation of this line in slope-intercept form? So any line can be represented in slope-intercept form, is y is equal to mx plus b, where this m right over here, that is of the slope of the line. And this b over here, this is the y-intercept of the line. Let me draw a quick line here just so that we can visualize that a little bit. So that is my y-axis. And then that is my x-axis. And let me draw a line. And since our line here has a negative slope, I'll draw a downward sloping line. So let's say our line looks something like that. So hopefully, we're a little familiar with the slope already. The slope essentially tells us, look, start at some point on the line, and go to some other point of the line, measure how much you had to move in the x direction, that is your run, and then measure how much you had to move in the y direction, that is your rise. And our slope is equal to rise over run. And you can see over here, we'd be downward sloping. Because if you move in the positive x direction, we have to go down. If our run is positive, our rise here is negative. So this would be a negative over a positive, it would give you a negative number. That makes sense, because we're downward sloping. The more we go down in this situation, for every step we move to the right, the more downward sloping will be, the more of a negative slope we'll have. So that's slope right over here. The y-intercept just tells us where we intercept the y-axis. So the y-intercept, this point right over here, this is where the line intersects with the y-axis. This will be the point 0 comma b. And this actually just falls straight out of this equation. When x is equal to 0-- so let's evaluate this equation, when x is equal to 0. y will be equal to m times 0 plus b. Well, anything times 0 is 0. So y is equal to 0 plus b, or y will be equal to b, when x is equal to 0. So this is the point 0 comma b. Now, they tell us what the slope of this line is. They tell us a line has a slope of negative 3/4. So we know that our slope is negative 3/4, and they tell us that the line goes through the point 0 comma 8. They tell us we go through the-- Let me just, in a new color. I've already used orange, let me use this green color. They tell us what we go through the point 0 comma 8. Notice, x is 0. So we're on the y-axis. When x is 0, we're on the y axis. So this is our y-intercept. So b, we could say-- we could do a couple-- our y-intercept is the point 0 comma 8, or we could say that b-- Remember, it's also 0 comma b. We could say b is equal to 8. So we know m is equal to negative 3/4, b is equal to 8, so we can write the equation of this line in slope-intercept form. It's y is equal to negative 3/4 times x plus b, plus 8. And we are done. Slope-intercept equation from two points A line goes through the points (-1, 6) and (5, -4). What is the equation of the line? Let's just try to visualize this. So that is my x axis. And you don't have to draw it to do this problem but it always help to visualize That is my y axis. And the first point is (-1,6) So (-1, 6). So negative 1 coma, 1, 2, 3, 4 ,5 6. So it's this point, rigth over there, it's (-1, 6). And the other point is (5, -4). So 1, 2, 3, 4, 5. And we go down 4, So 1, 2, 3, 4 So it's right over there. So the line connects them will looks something like this. Line will draw a rough approximation. I can draw a straighter than that. I will draw a dotted line maybe Easier do dotted line. So the line will looks something like that. So let's find its equation. So good place to start is we can find its slope. Remember, we want, we can find the equation y is equal to mx plus b. This is the slope-intercept form where m is the slope and b is the y-intercept. We can first try to solve for m. We can find the slope of this line. So m, or the slope is the change in y over the change in x. Or, we can view it as the y value of our end point minus the y value of our starting point over the x-value of our end point minus the x-value of our starting point. Let me make that clear. So this is equal to change in y over change in x wich is the same thing as rise over run wich is the same thing as the y-value of your ending point minus the y-value of your starting point. This is the same exact thing as change in y and that over the x value of your ending point minus the x-value of your starting point This is the exact same thing as change in x. And you just have to pick one of these as the starting point and one as the ending point. So let's just make this over here our starting point and make that our ending point. So what is our change in y? So our change in y, to go we started at y is equal to six, we started at y is equal to 6. And we go down all the way to y is equal to negative 4 So this is rigth here, that is our change in y You can look at the graph and say, oh, if I start at 6 and I go to negative 4 I went down 10. or if you just want to use this formula here it will give you the same thing We finished at negative 4, we finished at negative 4 and from that we want to subtract, we want to subtract 6. This right here is y2, our ending y and this is our beginning y This is y1. So y2, negative 4 minus y1, 6. or negative 4 minus 6. That is equal to negative 10. And all it does is tell us the change in y you go from this point to that point We have to go down, our rise is negative we have to go down 10. That's where the negative 10 comes from. Now we just have to find our change in x. So we can look at this graph over here. We started at x is equal to negative 1 and we go all the way to x is equal to 5. So we started at x is equal to negative 1, and we go all the way to x is equal to 5. So it takes us one to go to zero and then five more. So are change in x is 6. You can look at that visually there or you can use this formula same exact idea, our ending x-value, our ending x-value is 5 and our starting x-value is negative 1. 5 minus negative 1. 5 minus negative 1 is the same thing as 5 plus 1. So it is 6. So our slope here is negative 10 over 6. wich is the exact same thing as negative 5 thirds. as negative 5 over 3 I divided the numerator and the denominator by 2. So we now know our equation will be y is equal to negative 5 thirds, that's our slope, x plus b. So we still need to solve for y-intercept to get our equation. And to do that, we can use the information that we know in fact we have several points of information We can use the fact that the line goes through the point (-1,6) you could use the other point as well. We know that when is equal to negative 1, So y is eqaul to 6. So y is equal to six when x is equal to negative 1 So negative 5 thirds times x, when x is equal to negative 1 y is equal to 6. So we literally just substitute this x and y value back into this and know we can solve for b. So let's see, this negative 1 times negative 5 thirds. So we have 6 is equal to positive five thirds plus b. And now we can subtract 5 thirds from both sides of this equation. so we have subtracted the left hand side. From the left handside and subtracted from the rigth handside And then we get, what's 6 minus 5 thirds. So that's going to be, let me do it over here We take a common denominator. So 6 is the same thing as Let's do it over here. So 6 minus 5 over 3 is the same thing as 6 is the same thing as 18 over 3 minus 5 over 3 6 is 18 over 3. And this is just 13 over 3. And this is just 13 over 3. And then of course, these cancel out. So we get b is equal to 13 thirds. So we are done. We know We know the slope and we know the y intercept. The equation of our line is y is equal to negative 5 thirds x plus our y-intercept which is 13 which is 13 over 3. And we can write these as mixed numbers. if it's easier to visualize. 13 over 3 is four and 1 thirds. So this y-intercept right over here. this y-intercept right over here. That's 0 coma 13 over 3 or 0 coma 4 and 1 thirds. And even with my very roughly drawn diagram it those looks like this. And the slope negative 5 thirds that's the same thing as negative 1 and 2 thirds. You can see here the slope is downward because the slope is negative. It's a little bit steeper than a slope of 1. It's not quite a negative 2. It's negative 1 and 2 thirds. if you write this as a negative, as a mixed number. So, hopefully, you found that entertaining. Constructing linear equations from context Tara was hiking up a mountain. She started her hike at an elevation of 1,200 meters and ascended at a constant rate. After four hours, she reached an elevation of 1,700 meters. Let y represent Tara's elevation in meters after x hours. And they ask us, and this is from an exercise on Khan Academy, it says, complete the equation for the relationship between the elevation and the number of hours. And if you're on Khan Academy, you would type it in, but we can do it by hand. So pause this video and work it out on some paper and let's see if we get to the same place. All right, now let's do this together. So first of all, they tell us that she's ascending at a constant rate. So that's a pretty good indication that we could describe her elevation based on the number of hours she travels with a linear equation. And we could even figure out that constant rate. It says that she goes from 1,200 meters to 1,700 meters in four hours. So we could say her rate is going to be her change in elevation over a change in time. So her change in elevation is 1,700 meters minus 1,200 meters and she does this over four hours. Over, her change in time is four hours. So her constant rate in the numerator here, 1,700 minus 1,200 is 500 meters. She's able to go up 500 meters in four hours. If we divide 500 by four, this is 125 meters per hour. And so we could use this now to think about what our equation would be. Our elevation y would be equal to, well, where is she starting? Well, it's starting at 1,200 meters. So she's starting at 1,200 meters. And then to that, we're going to add how much she climbs based on how many hours she's traveled. So it's going to be this rate, 125 meters per hour times the number of hours she has been hiking. So the number of hours is x times x. So this right over here is an equation for the relationship between the elevation and the number of hours. Another way you could have thought about it, you could have said, okay, this is going to be a linear equation because she's ascending at a constant rate. You could say the slope intercept form for a linear equation is y is equal to mx plus b, where b is your y-intercept. What is the value of y when x is equal to zero? And you'd say, all right, when x is equal to zero, she's at an elevation of 1,200. And then m is our slope. So that's the rate at which our elevation is increasing. And that's what we calculated right over here. Our slope is 125 meters per hour. So notice, these are equivalent. I just have, these two terms are swapped. So we could either write y is equal to 1,200 plus 125x or you could write it the other way around. You could write 125x plus 1,200. They are equivalent.