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8th grade (Eureka Math/EngageNY)
Course: 8th grade (Eureka Math/EngageNY) > Unit 4
Lesson 3: Topic C: Slope and equations of lines xintercept of a line
 Intercepts from a table
 Intercepts from a graph
 Intercepts from an equation
 Worked example: intercepts from an equation
 Intercepts from an equation
 Slope & direction of a line
 Intro to slope
 Slope formula
 Worked example: slope from graph
 Slope of a line: negative slope
 Slope from graph
 Worked example: slope from two points
 Slope of a horizontal line
 Slope from two points
 Intro to slopeintercept form
 Slopeintercept intro
 Slope from equation
 Graph from slopeintercept equation
 Graph from slopeintercept form
 Slopeintercept equation from graph
 Slopeintercept equation from graph
 Slopeintercept equation from slope & point
 Slopeintercept equation from two points
 Slopeintercept from two points
 Slopeintercept form from a table
 Converting to slopeintercept form
 Slopeintercept form problems
 Proving slope is constant using similarity
 Intro to pointslope form
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Slope formula
CCSS.Math:
Learn how to write the slope formula from scratch and how to apply it to find the slope of a line from two points.
It's kind of annoying to have to draw a graph every time we want to find the slope of a line, isn't it?
We can avoid this by writing a general formula for slope. Before we start, let's remember how slope is defined:
Let's draw a line through two general points left parenthesis, start color #1fab54, x, start subscript, 1, end subscript, end color #1fab54, comma, start color #e07d10, y, start subscript, 1, end subscript, end color #e07d10, right parenthesis and left parenthesis, start color #1fab54, x, start subscript, 2, end subscript, end color #1fab54, comma, start color #e07d10, y, start subscript, 2, end subscript, end color #e07d10, right parenthesis.
An expression for start color #1fab54, start text, c, h, a, n, g, e, space, i, n, space, x, end text, end color #1fab54 is start color #1fab54, x, start subscript, 2, end subscript, minus, x, start subscript, 1, end subscript, end color #1fab54:
Similarly, an expression for start color #e07d10, start text, c, h, a, n, g, e, space, i, n, space, y, end text, end color #e07d10 is start color #e07d10, y, start subscript, 2, end subscript, minus, y, start subscript, 1, end subscript, end color #e07d10:
Now we can write a general formula for slope:
That's it! We did it!
Using the slope formula
Let's use the slope formula to find the slope of the line that goes through the points left parenthesis, 2, comma, 1, right parenthesis and left parenthesis, 4, comma, 7, right parenthesis.
Step 1: Identify the values of x, start subscript, 1, end subscript, x, start subscript, 2, end subscript, y, start subscript, 1, end subscript, and y, start subscript, 2, end subscript.
y, start subscript, 2, end subscript, equals, 7, space, space, space, space, space, space, space, space
Step 2: Plug in these values to the slope formula to find the slope.
Step 3: Gut check. Make sure this slope makes sense by thinking about the points on the coordinate plane.
Yup! This slope seems to make sense since the slope is positive, and the line is increasing.
Using the slope formula walkthrough
Let's use the slope formula to find the slope of the line that goes through the points left parenthesis, 6, comma, minus, 3, right parenthesis and left parenthesis, 1, comma, 7, right parenthesis.
Step 1: Identify the values of x, start subscript, 1, end subscript, x, start subscript, 2, end subscript, y, start subscript, 1, end subscript, and y, start subscript, 2, end subscript.
Step 2: Plug in these values to the slope formula to find the slope.
Step 3: Gut check. Make sure this slope makes sense by thinking about the points on the coordinate plane.
Let's practice!
Something to think about
What happens in the slope formula when x, start subscript, 2, end subscript, equals, x, start subscript, 1, end subscript?
As a reminder, here is the slope formula:
Feel free to discuss in the comments below!
Want to join the conversation?
 Why is the slope formula y/x? Why not yx or y+x?
Thanks to anyone who answers.
Jack.(21 votes) Slope is something that is also referred to as the rate of change. For example, if you had a savings account that you deposited no money into initially but you deposit 20$ weekly, your rate of change, or slope for this problem would be 20. This is because your xvalue in this situation would be the number of weeks passed since you have created your bank account, and the yvalue is how much money you have deposited into your account, fully. Since you are looking at the rate of change between the weeks, you divide the change in y per week, 20, by 1 for the number of weeks. I hope this somewhat answers your question.(47 votes)
 Something To Think About
I think that when x_2 = x_1 then the slope will become undefined because x_2  x_1 equals zero. Therefore when you divide y_2  y_1 it won't be possible.
Example
(5,10) (5,15)
x_1 = 5
x_2 = 5
y_1 =10
y_2 =15
5  5= 0
1510=5
5/0= Undefined(33 votes) Yes, you are correct. The slope of any line through two different points with the same xcoordinate (that is, a vertical line) is always undefined, for the reason you stated.(13 votes)
 I think that when X2 = X1, the slope is undefined(17 votes)
 Yes! That is correct.(6 votes)
 If I Get The Right Answer Then Why Do I Have To Simplify?(5 votes)
 Simplifying just makes it easier to read/understand. It makes it more "simple." Although both are equal, it is just easier to work with if it's simplified afterwards.(7 votes)
 For the last one, if x_1 equals x_2 it is undefined, this is because, from the other videos, it was said that if the two points have the same x when drawing the line, it will be straight up and down, with no slope, but those are called undefined as there really is an undefined slope to it(5 votes)
 Yes you are correct that the slope is undefined if x_1 = x_2. Good job!(2 votes)
 bro im honestly so gone(5 votes)
 Slope is basically the change in the y direction divided by the change in the x direction. If you don't know graphs, you might want to learn that first.(2 votes)
 Something to Think About:
When x1 = x2, it means that x1x2=0. So therefore the formula will simplify to y/0. And as x/0 is undefined, the slope should also be undefined.(5 votes)  When x2=x1 then the slope will always be undefined because (y2y1)/0 will always be undefined(5 votes)
 In the very last part, why is the formula for slope delta y/delta x instead of delta x/ delta y?(2 votes)
 This is because the equation that describes a line is y=mx+c.
If we have the y and x values (as in the coordinates), and c is constant for both points (which if it is two point on one line, we know it is) than we can solve for m with algebra.
If we have two coordinates on a line (x1,y1 =1,2) and (x2, y2 =3,6) we can solve for m as follows.
(x2,y2) 6=m3+c

(x1,y1) 2=m1+c
1st step: cc =0
we are left with
6=m3

2=m1
The first equation minus the second =
4=2m
But we want the slope (m) on one side so we can solve for M.
4/2=m
2=m which is your slope
What you have done here is take y2 from y1 on the left, x2 from x1 on the right, then divided by x to get m on its own. We can do this in one step instead to get the slope by the equation
(y2y1)/(x2x1)=m
That is why you divide by x rather than Y.(7 votes)
 in the formula mx+b=y, I understand that "m" is the slope and "b" is the yintercept, but what is x and y?(3 votes)
 when you graph the line, mx+b=y and fill in the slope and yintercept, the x and y represent points that are on the line that you graphed. For example, if the equation was 5x+10=y, you could create pairs of (x,y) coordinates by plugging in numbers for x and y. In this case, if x was 5, y would be 35 or vice versa. Based on this, you could say that (5,35) is a point on the line, 5x+10=y.(4 votes)