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Writing and using formulae

Writing and using formulae

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Summary

Writing and using formulae

In a nutshell

A formula can be created by analysing a question given in context. Questions can be based on calculations of physical quantities or geometrical measures.

Note: Remember to test the formula to see if it works, by substituting in values.



Making a formula

To make a formula, read the information given, give the unknown quantity a letter name and use your knowledge of algebra to give the formula.


Example 1

Write a formula for the cost, c, of n boxes of chocolate at £2 each.


c=2nc=2n

If there are 3 boxes, n=3n=3, so

c=2n=2×3=6c=2n = 2 \times 3 = 6

So the total cost is £6\underline{£6}.

Example 2

Write a formula for the age of Fred, f, who is four years older than George, g.


You can write the formula is 2 different ways, Fred's age is four more than George's age, or George's age is four less than Fred's age. 

f=g+4\underline {f=g+4} or g=f4\underline{g=f-4}


Example 3

A restaurant charges £C£C for delivering pizzas, through a delivery app. They charge a standard fee of £1.50£1.50 and an additional charge of 80p80p for each mile, m, travelled. Write a formula for CC.​


C=0.8m+1.5\underline{C=0.8m+1.5}

Note: Remember to convert to the same units if necessary, in this case 80p80p has been converted to £0.8£0.8.


Example 4

A rectangle has length 4cm4cm more than width. Write a formula for the area.



x+4x+4​​

xx​​
Maths; Formulae and equations; KS3 Year 7; Writing and using formulae



A=width×lengthA=x(x+4)\begin {aligned}A&= width \times length \\A&= x(x+4) \\ \end {aligned}

A=x2+4x\underline {A= x^2 + 4x}​​

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FAQs - Frequently Asked Questions

Is there more than one way of writing a formula in algebra?

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