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Slope formula

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:
start text, S, l, o, p, e, end text, equals, start fraction, start color #e07d10, start text, C, h, a, n, g, e, space, i, n, space, y, end text, end color #e07d10, divided by, start color #1fab54, start text, C, h, a, n, g, e, space, i, n, space, x, end text, end color #1fab54, end fraction
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:
start text, S, l, o, p, e, end text, equals, start fraction, start color #e07d10, start text, C, h, a, n, g, e, space, i, n, space, y, end text, end color #e07d10, divided by, start color #1fab54, start text, C, h, a, n, g, e, space, i, n, space, x, end text, end color #1fab54, end fraction, equals, start fraction, start color #e07d10, y, start subscript, 2, end subscript, minus, y, start subscript, 1, end subscript, end color #e07d10, divided by, start color #1fab54, x, start subscript, 2, end subscript, minus, x, start subscript, 1, end subscript, end color #1fab54, end fraction
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.
x, start subscript, 1, end subscript, equals, 2
y, start subscript, 1, end subscript, equals, 1
x, start subscript, 2, end subscript, equals, 4
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.
start text, S, l, o, p, e, end text, equals, start fraction, y, start subscript, 2, end subscript, minus, y, start subscript, 1, end subscript, divided by, x, start subscript, 2, end subscript, minus, x, start subscript, 1, end subscript, end fraction, equals, start fraction, 7, minus, 1, divided by, 4, minus, 2, end fraction, equals, start fraction, 6, divided by, 2, end fraction, equals, 3
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.
x, start subscript, 1, end subscript, equals
y, start subscript, 1, end subscript, equals
x, start subscript, 2, end subscript, equals
y, start subscript, 2, end subscript, equals

Step 2: Plug in these values to the slope formula to find the slope.
start text, S, l, o, p, e, end text, equals, start fraction, y, start subscript, 2, end subscript, minus, y, start subscript, 1, end subscript, divided by, x, start subscript, 2, end subscript, minus, x, start subscript, 1, end subscript, end fraction, equals

Step 3: Gut check. Make sure this slope makes sense by thinking about the points on the coordinate plane.
Does this slope make sense?

Let's practice!

1) Use the slope formula to find the slope of the line that goes through the points left parenthesis, 2, comma, 5, right parenthesis and left parenthesis, 6, comma, 8, right parenthesis.

2) Use the slope formula to find the slope of the line that goes through the points left parenthesis, 2, comma, minus, 3, right parenthesis and left parenthesis, minus, 4, comma, 3, right parenthesis.

3) Use the slope formula to find the slope of the line that goes through the points left parenthesis, minus, 5, comma, minus, 7, right parenthesis and left parenthesis, minus, 2, comma, minus, 1, right parenthesis.

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:
start text, S, l, o, p, e, end text, equals, start fraction, y, start subscript, 2, end subscript, minus, y, start subscript, 1, end subscript, divided by, x, start subscript, 2, end subscript, minus, x, start subscript, 1, end subscript, end fraction
Feel free to discuss in the comments below!

Want to join the conversation?

• I think that when X2 = X1, the slope is undefined
• Yes! That is correct.
• 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
• Yes you are correct that the slope is undefined if x_1 = x_2. Good job!
• Using the slope​ formula, find the slope of the line through the points (0,0) and(3,6) . Use pencil and paper. Explain how you can use mental math to find the slope of the line. The slope of the line is enter your response here. ​(Type an integer or a simplified​ fraction.)
• 0s make it easy because you end up with a proportional relationship where y/x = 6/3 or when you reduce and multiply by x, you get y=2x. Using the slope formula, m= (6-0)/(3-0) which is just m=6/3=2.
When x1 = x2, it means that x1-x2=0. So therefore the formula will simplify to y/0. And as x/0 is undefined, the slope should also be undefined.
• In the very last part, why is the formula for slope delta y/delta x instead of delta x/ delta y?
(1 vote)
• 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: c-c =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

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
(y2-y1)/(x2-x1)=m
That is why you divide by x rather than Y.
• in the formula mx+b=y, I understand that "m" is the slope and "b" is the y-intercept, but what is x and y?
• when you graph the line, mx+b=y and fill in the slope and y-intercept, 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.
• I am assuming that since there is no y variable in said equation that the slope would just be undefined. i don't fully understand it so it would be nice to have an explanation.

• Why is the slope formula y/x? Why not y-x or y+x?