Direct Proportion

Maths: Edexcel A Level Pure Maths, topic 2.5.9

Direct Proportion

Two variables are directly proportional to one another if the size of one variable increases when the size of other increases.

Directly proportional

Directly proportional

If $x$ and $y$ are directly proportional to one another, we write:
$x\large{\propto} y$
Where $\large \propto$ is the symbol of proportionality.
We can always write the equation:
$x = ky$
Where $k$ is called the constant of proportionality and is a non-zero number.

Proportional

Proportional

For example, the area of a square is proportional to the length of one side squared, so we can write
$A\large{\propto} x^2 \rightarrow A=x^2$
In this case, the constant of proportionality $k$ is equal to 1.

Test yourself on Direct Proportion

y $\alpha$ x
What does this mean?
What does the ' $\alpha$ ' symbol mean in maths?
Which of the following means 'a is proportional to b³'?
Which of the following symbols means 'proportional to' in maths?

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