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GCSE Maths Ratio and Proportion Proportion

Direct Proportion Formula

Direct Proportion Formula

Here we will learn about direct proportion formulas, including what the proportion formulas are and how to interpret them.

There are also direct proportion formula worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

What is the direct proportion formula?

The direct proportion formula is an algebraic formula which represents the directly proportional relationship between two variables.

Direct proportion is one type of a proportionality relationship. As one value increases, so does the other value.

The proportionality symbol is . If the variables x and y are directly proportional then we can write the relationship using the proportionality symbol.

y\propto x

We can also write this as a formula using the constant of proportionality, k.

k is a constant value that links the two variables.

y=kx

Proportional relationships can also be represented by graphs.  

The following is a direct proportion graph.

Direct proportion formula image 1.

It is a straight line graph going through the origin.

Direct proportion formulas can involve powers and roots. If the variables were x and y, and y is directly proportional to x^{2}, we can write the relationship using the proportionality symbol.

y\propto x^2

We can also write this as a formula using the constant of proportionality k.

y=kx^2

What is the direct proportion formula?

What is the direct proportion formula?

How to recognise a direct proportion formula

In order to recognise a direct proportion formula:

  1. Look for an equation in the form y=kx.
  2. Check there are no other terms in the equation.

Explain how to recognise a direct proportion formula

Explain how to recognise a direct proportion formula

Direct proportion worksheet (includes direct proportion formula)

Direct proportion worksheet (includes direct proportion formula)

Direct proportion worksheet (includes direct proportion formula)

Get your free direct proportion formula worksheet of 20+ direct proportion questions and answers. Includes reasoning and applied questions.

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Direct proportion worksheet (includes direct proportion formula)

Direct proportion worksheet (includes direct proportion formula)

Direct proportion worksheet (includes direct proportion formula)

Get your free direct proportion formula worksheet of 20+ direct proportion questions and answers. Includes reasoning and applied questions.

DOWNLOAD FREE

Direct proportion formula examples

Example 1: recognise the direct proportion formula

Which of these equations indicate that y\propto x?

A B C D

y=x+5 \hspace{1cm} y=\frac{5}{x} \hspace{1cm} y=1-5x \hspace{1cm} y=5x

  1. Look for an equation in the form y=kx.

We are looking for a direct proportion equation. We are looking for an equation with the variable, x, being multiplied by a number.

We can eliminate equation B as this has a \frac{1}{x} term, this would be inverse proportion.

2Check there are no other terms in the equation.

For direct proportion there can be no addition or subtraction involved in the equation.

So we can eliminate equations A and C.

The only equation which only has an x -term being multiplied by a number is equation D.

y=5x indicates that y \propto x.

Example 2: recognise the direct proportion formula

Which of these equations indicate that y\propto x?

A B C D

y=\frac{4}{x} \hspace{1cm} y=4x \hspace{1cm} y=4x-3 \hspace{1cm} y=4x+2

We are looking for a direct proportion equation. We are looking for an equation with the variable, x, being multiplied by a number.


We can eliminate equation A as this has a \frac{4}{x} term, this would be inverse proportion.

For direct proportion there can be no addition or subtraction involved in the equation.


So we can eliminate equations C and D.


The only equation that only has an x -term being multiplied by a number is equation B.


y=4x indicates y\propto x.

How to construct a direct proportion formula

In order to construct a direct proportion formula:

  1. Write down the general proportion formula.
  2. Find the constant \textbf{k} , the constant of proportionality.
  3. Write down the equation.

Explain how to construct a direct proportion formula

Explain how to construct a direct proportion formula

Constructing direct proportion formula examples

Example 3: constructing a direct proportion equation

y is directly proportional to x.

When y=35, \ x=5.

Find a formula for y in terms of x.

From the first sentence we can write down the relationship as


y\propto x.


We can then write this as a formula using the constant of proportionality k,


y=kx .


Where k is the constant of proportionality.

We can find the constant of proportionality by substituting in the corresponding values we are given.


We are told ‘when y=35, \ x=5’.


\begin{aligned} y&= kx \\\\ 35 &= k \times 5 \\\\ k&=35 \div 5=7 \end{aligned}

To finish off we can write the direct proportion equation as


y=7x.

Example 4: constructing a direct proportion equation

y is directly proportional to x.

When y=4, \ x=20.

Find a formula for y in terms of x.

From the first sentence we can write down the relationship as


y\propto x.


We can then write this as a formula using the constant of proportionality k,


y=kx .


Where k is the constant of proportionality.

We can find the constant of proportionality by substituting in the corresponding values we are given.


We are told ‘when y=4, \ x=20’.


\begin{aligned} y&= kx \\\\ 4 &= k \times 20 \\\\ k&=4 \div 20=0.2 \end{aligned}

To finish off we can write the direct proportion equation as


y=0.2x \ \text{or} \ y=\frac{x}{5}.

Example 5: constructing a direct proportion equation involving a power or root

y is directly proportional to \sqrt{x}.

When y=40, \ x=4.

Find a formula for y in terms of x.

From the first sentence we can write down the relationship as


y\propto \sqrt{x}.


We can then write this as a formula using the constant of proportionality k,


y=k\sqrt{x} .


Where k is the constant of proportionality.

We can find this unknown value, the constant of proportionality by substituting in the corresponding values we are given.


We are told ‘when y=40, \ x=4’.


\begin{aligned} y&= k\sqrt{x} \\\\ 40 &= k\times \sqrt{4} \\\\ 40 &= k\times 2 \\\\ k&=40 \div 2=20 \end{aligned}

To finish off we can write the direct proportion equation as


y=20\sqrt{x}.

Common misconceptions

  • Confusing direct proportion and inverse proportion

You will need to learn which formula is for which type of proportionality.

Direct proportionInverse proportion
y\propto x y\propto \frac{1}{x}
y=kx y=\frac{k}{x}

  • Not recognising and applying any powers or roots

Most direct proportional formulas just involve x, but they can involve powers such as x^{2} or roots such as \sqrt{x}.

  • Assuming the constant of proportionality can only be an integer

The constant of proportionality can be an integer (a whole number), but they can also be decimals or fractions.

For example,

y=\frac{x}{2}

Even though there is a fraction, the constant of proportionality is \frac{1}{2} or 0.5.

y=\frac{x}{2}=\frac{1}{2} \times x=0.5 \times x

Practice direct proportion formula questions

1. Which of these equations does NOT indicate y \propto x?

y=2x
GCSE Quiz False

y=x+2
GCSE Quiz True

y=\frac{x}{2}
GCSE Quiz False

y=20x
GCSE Quiz False

For direct proportion there can be no addition or subtraction involved in the equation. The incorrect equation is y=x+2.

2. Which of these equations indicate y \propto x?

y=x+5
GCSE Quiz False

y=5x
GCSE Quiz True

y=\frac{5}{x}
GCSE Quiz False

y=5x^2
GCSE Quiz False

For direct proportion there can be no addition or subtraction involved in the equation. The correct equation is y=5x.

3. Which of these equations indicates direct proportion?

y=4-10x
GCSE Quiz False

y=10x+4
GCSE Quiz False

y=\frac{10}{x^2}
GCSE Quiz False

y=10x
GCSE Quiz True

For direct proportion there can be no addition or subtraction involved in the equation. The correct equation is y=10x.

4. y is directly proportional to x.

When y=24, \ x=8.

Find a formula for y in terms of x.

y=4x
GCSE Quiz False

y=\frac{3}{x}
GCSE Quiz False

y=\frac{x}{4}
GCSE Quiz False

y=3x
GCSE Quiz True

Write down the relationship between y and x and substitute the given values to find k, the constant of proportionality.

 

\begin{aligned} y&\propto x \\\\ y&= kx \\\\ 24 &= k\times 3 \\\\ k&=24\div 3=8 \end{aligned}

 

So, the equation is

 

y=8x.

5. y is directly proportional to x.

When y=10, \ x=5.

Find a formula for y in terms of x.

y=50x
GCSE Quiz False

y=2x
GCSE Quiz True

y=\frac{x}{2}
GCSE Quiz False

y=\frac{2}{x}
GCSE Quiz False

Write down the relationship between y and x and substitute the given values to find k, the constant of proportionality.

 

\begin{aligned} y&\propto x \\\\ y&= kx \\\\ 10&= k\times 5 \\\\ k&=10\div 5=2 \end{aligned}

 

So, the equation is

 

y=2x.

6. y is directly proportional to x^{2}.

When y=18, \ x=3.

Find a formula for y in terms of x^{2}.

y=6x
GCSE Quiz False

y=2x^2
GCSE Quiz True

y=6x^2
GCSE Quiz False

y=3x
GCSE Quiz False

Write down the relationship between y and x and substitute the given values to find k, the constant of proportionality.

 

\begin{aligned} y&\propto x^2 \\\\ y&= kx^2 \\\\ 18&= k\times 3^2 \\\\ 18&= 9k \\\\ k&=18\div 3=2 \end{aligned}

 

So, the equation is

 

y=2x^2.

Direct proportion formula GCSE questions

1. y is directly proportional to x and k is a constant.

Identify the correct equation.

 

Equation A –  y=\frac{k}{x}

 

Equation B –  y=kx

 

Equation C –  y=k-x

 

Equation D –  y=\frac{x}{k}

(1 mark)

Show answer

Equation B

(1)

2. y is proportional to x.

When y=36, \ x=12.

 

(a) Find the equation for y in terms of x.

 

(b) Find the value of x when y=50.

 

(4 marks)

Show answer

(a)

 

\begin{aligned} y&=kx \\\\ 36&=k \times 12 \\\\ k&=36 \div 12=3 \end{aligned}

(1)

y=3x

(1)

 

(b)

 

50=3x \ so \ x=50 \div 3

(1)

x=\frac{50}{3}=16\frac{2}{3}

(1)

3. m is directly proportional to p .

When p=5, \ m=20.

Find m when p=3.

 

(3 marks)

Show answer

m=kp \ or \ 20=k \times 5

(1)

k=4 \ or \ m=4p

(1)

m=12

(1)

Learning checklist

You have now learned how to:

  • Recognise a direct proportion formula
  • Construct a direct proportion equation
  • Model situations by translating them into algebraic formulae

The next lessons are

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