In Graphing Proportional Relationships we see how the slope of the line generated when a linear relationship is plotted represents the unit rate e.g. miles/ hour, cost/ mile, etc. The slope of a line can be represented using a positive or negative number to show its steepness and direction. The steepness is sometimes referred to as the rate of change.
The table and graph below shows x and y values resulting from the function y = 2x.
The slope of the line above is 2. Each value of y is 2 times the corresponding value of x.
Look at the equations and resulting tables and graphs below.
Notice also that in the above examples the line slopes up from left to right. These are positive slopes show that as x increases, so does y.
The examples below show that as x increases, y decreases. This results in a negative slope that runs downwards from left to right.
Enter slope(m) and y-intercept(b) below then click Draw Line
Drawing equation y = 2x – 1
How To Calculate Slope
We have seen already that if a linear relationship is given as an equation in the form, y = mx + b, then the constant m represents the slope. Often though a linear relationship is given as sets of related values that can be graphed as ordered pairs. See the example below:
If you know any two points on a line you can calculate the slope:
Slope = difference in y-values/ difference in x-values (rise/ run)
Slope = (y2 – y1)/ (x2 – x1)
Using points (-3,-3) and (1,5) from the table above we get:
Slope = (5 – (-3))/ (1 – (-3))
Slope = 8/4
Slope = 2
You will find more examples showing the use of the slope formula here.
Any 2 Points Will Do – Similar Triangles
As the example above shows, slope can be calculated using the co-ordinate values of any two points on the line. Note below how any two pairs of points (that’s two sets of two!) on the same line form similar triangles on the coordinate grid. The angles forming the slope in similar triangles are the same in each triangle. There is more on similar triangles and their geometric properties here.
Look at points A and B. They form triangle BAE on the grid. Points C and D form triangle DCG. △BAE and △DCG are similar triangles as their three internal angles match either other. Each of these two triangles are similar to △DBF. In fact any right triangle with its hypotenuse lying on the slope line would be similar.
Use the worksheet(s) below for practice.