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How to find the slope of a line.

Worksheet on Slope Of A Line
Slope Applet (html5)
slope applet
Slope Formula Calculator

(Free online tool calculates slope given 2 points)
The slope of a line characterizes the general direction in which a line points.  To find the slope, you divide the difference of the y-coordinates of a point on a line by the difference of the x-coordinates.
slope of a line

 Formula to find the slope of a line  

Formula for the slope of a line
Example One
The slope of a line going through the point (1,2) and the point (4,3) is 1/3.
Graph of the slope of a line
Example 2 of the Slope of A line
The slope of a line through the points (3, 4) and (5, 1) is -3/2 because every time that the line goes down by 3(the change in y or the rise) the line moves to the right (the run) by 2.
Picture of the  slope of a line
confused

Does it matter which point you start with?

There is only one way to know! Let's try to find the slope of a line through the points (4,3) and (1,2) .First we'll start with one point and then we'll start with the other.

First, let's start with the point (4,3)
$$slope= \frac{y_{2}-y_{1}}{x_{2}-x_{1}} = \frac{3-2}{4-1} =\frac{1}{3} $$

Now, let's start with the point (1,2)

$$slope= \frac{y_{2}-y_{1}}{x_{2}-x_{1}} = \frac{2-3}{1-4} =\frac{-1}{-3}=\frac{1}{3} $$

And the Answer is...

It does not matter which point you put first. You can start with (4,3) or with (1,2) and, either way, you end with the exact same number! $\frac{1}{3} $

Video Tutorial on the Slope of a Line

Slope of vertical and horizontal lines


  • The slope of a vertical line is undefined

    . This is because any vertical line has a $$\Delta x$$ or "run" of zero. Whenever zero is the denominator of the fraction in this case of the fraction representing the slope of a line, the fraction is undefined. The picture below shows a vertical line (x=1)
  • The slope of a horizontal line is zero

    This is because any horizontal line has a $$\Delta y$$ or "rise" of zero. Therefore, regardless of what the run is (provided its' not also zero!), the fraction representing slope has a zero in its numerator. Therefore, the slope must evaluate to zero. Below is a picture of a horizontal line--you can see that it does not have any 'rise' to it.
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Do any two points on a line have the same slope?

Slope is consistent

And the answer is...

Yes, and this is a fundamental point to remember about calculating slope.
Every line has a consistent slope. In other words, the slope of a line never changes. This fundamental idea means that you can choose ANY two points on a line to find the slope. This should intuitively make sense with your own understanding of a straight line. After all, if the slope of a line could change, then it would be a zigzag line and not a straight line, as you can see in the picture below.

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Practice Problems
WARNING! Can you catch the error in the following problem Jennifer was trying to find the slope that goes through the points $$(\color{blue}{1},\color{red}{3})$$ and $$ (\color{blue}{2}, \color{red}{6})$$ . She was having a bit of trouble applying the slope formula, tried to calculate slope 3 times, and she came up with 3 different answers. Can you determine the correct answer?
Answer
The correct answer is the Second attempt.


The problem with her first attempt was that she did not consistently use the points. What she did, in attempt one, was used $$ \frac{\color{red}{y{\boxed{_2}}-y_{1}}}{\color{blue}{x\boxed{_{1}}-x_{2}}} $$

The problem with her third attempt was that she did switched the rise and run. In other words, she put the x values on the top and the y values on the bottom which, of course, is not how you do slope! $$ \cancel {\frac{\color{blue}{x_{2}-x_{1}}}{\color{red}{y_{2}-y_{1}}}} $$
First Attempt at Slope
$$ slope= \frac{rise}{run} \\= \frac{\color{red}{y_{2}-y_{1}}}{\color{blue}{x_{2}-x_{1}}} \\= \frac{6-3}{1-2} \\= \frac{3}{-1} =\boxed{-3} $$
Second Attempt at Slope
$$ slope= \frac{rise}{run} \\= \frac{\color{red}{y_{2}-y_{1}}}{\color{blue}{x_{2}-x_{1}}} \\= \frac{6-3}{2-1} \\= \frac{3}{1} = \boxed{3} $$
Third Attempt at Slope
$$ slope= \frac{rise}{run} \\= \frac{\color{red}{y_{2}-y_{1}}}{\color{blue}{x_{2}-x_{1}}} \\ =\frac{2-1}{6-3} \\ =\boxed{ \frac{1}{3}} $$
Find the slope of A line Given Two Points

Practice Problem 5) What is the slope of a line that goes through the points (10,3) and (7 , 9) ?

Answer


Practice Problem 6) A line passes through (4, -2) and (4 , 3). What is its slope?

Answer



Practice Problem 7) A line passes through (2, 10) and (8 , 7). What is its slope?

Answer


Practice Problem 8) A line passes through (7,3) and (8 , 5). What is its slope?

Answer


Practice Problem 9) A line passes through (12,11) and (9 , 5). What is its slope?

Answer



Practice Problem 10) What is the slope of a line that goes through (4, 2) and (4, 5)?

Answer
Slope Practice -- Problem Generator

You can practice solving this sort of problem as much as you would like with the slope problem generator below. It will randomly generate numbers and ask for the slope of the line through those two points. You can chose how large the numbers will be by adjusting the difficulty level.
Difficulty Level (Determines how large the numbers are)
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Answer


Generate New Slope Problem