How to find the slope of a line.

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.

 Formula to find 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.

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.

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.
  

Do any two points on a line have the same slope?

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.

Interactive Slope

Click and drag around either of the points below, as you move the points around, you can see the slope of the line adjust accordingly.(Applet all alone)

Practice Problems
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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