We have learnt that the equation of a straight line has the form , where and are variables and and are constants.
In the case where we are left with the equation .
When then and are directly proportional to each other.
Being in direct proportion means the value of and increase or decrease by the same factor. If is doubled then is also doubled. If is reduced by a third then reduces by a third.
Let’s look at an example. . The values of and for to are shown below.
If is doubled from to then is doubled from to .
If is increased by a factor of from to then is increased by the same factor from to .
Graphs of go through the origin.