Unit 14 Section 2 : Plotting Points on Straight Lines
In this section we will be plotting points that lie on a straight line.
We will then look for relationships between the x and y coordinates of the points on the line.
Example
Below is a graph showing the points ( 1 , 3 ) , ( 2 , 4 ) , ( 3 , 5 ) , ( 4 , 6 ) , ( 5 , 7 )
We can then draw a straight line through all of the points:
Note how the line goes all the way across the graph, not just between the points.
Equation of a line
We can find an equation which describes the relationship between the x and y coordinates on the graph.
Looking at the pairs of coordinates, we can see that the yvalue is always two more than the xvalue.
The equation to represent this is:
y = x + 2 
(the ycoordinate can always be found by adding 2 to the xcoordinate) 
Note that this works for every point on the line, even with negative numbers and decimals.
Examples of other points on this line are: ( 0 , 2 ) , ( –1 , 1 ) , ( –4 , –2 ) and ( 2.5 , 4.5 )
Practice Question

Work out the answer to the question then click on the button marked
to see whether you are correct.
The points ( –1 , –3 ) , ( 0 , 0 ) , ( 1 , 3 ) and ( 2 , 6 ) have been plotted.
A straight line has been drawn through the four points.
How are the x and y coordinates of the points on the line related?
What is the equation of the line?

Exercises
Work out the answers to the questions below and fill in the boxes. Click on the
button to find out whether you have answered correctly. If you are right
then will appear and you should move on to the next
question. If appears then your answer is wrong. Click
on to clear your original answer and have another go.
If you can't work out the right answer then click on
to see
the answer.
NOTE: In these questions you need to plot points and draw lines on graphs.
To plot a point, just click on the graph with your left mouse button. To draw a line, hold down
the left mouse button at one point on the line and drag the pointer to another point on the line.
When you let go of the button the line will appear and it will automatically cross the whole graph.
If you make a mistake, press the delete key and the graph will be cleared.


Question 1
Question 2
Question 3
Question 4
Question 5
Question 6
Question 7
Question 8
You have now completed Unit 14 Section 2
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Produced by A.J. Reynolds May 2008
