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Understand and use formulae in KS3 algebra. Substitute values, rearrange to change the subject, and check results with sensible units in real-life contexts.
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Fascinating Fact:
Fuel economy links distance and fuel: litres per 100 km equals 100 × litres ÷ km. Example: 5 litres over 80 km is 6.25 litres per 100 km.
In KS3 Maths, formulae describe relationships between quantities, such as area A = l×w or speed v = d÷t. You will substitute numbers, use correct units, and rearrange a formula to make a chosen variable the subject.
Key Terms
Formula: A rule written using letters and numbers, e.g., v = d÷t.
Subject of a formula: The variable that is on its own, e.g., making t the subject gives t = d÷v.
Substitution: Replacing letters with given numbers to calculate a result.
Frequently Asked Questions (Click to see answers)
How do I rearrange a formula to make a different letter the subject?
Use inverse operations on both sides. For v = d÷t, multiply by t then divide by v to get t = d÷v.
What is the difference between an expression, an equation, and a formula?
An expression has no equals sign. An equation states two sides are equal. A formula is an equation showing a rule between variables.
How do I substitute values correctly in a formula?
Put numbers in place of letters with brackets where needed, use the correct order of operations, and include units in your final answer.
You are given the formula x - 2 = c and you are told that the value of c is 28. What is the value of x?
26
28
30
32
30 - 2 = c. Sometimes you have to work backwards in order to find the answer
2 .
You are given the formula a = 2b and you are told that the value of b is 4. What is the value of a?
2
4
6
8
2b = 2 x 4. We have been told that a = 2b and therefore a must be 8
3 .
The perimeter of a rectangle is twice its length plus twice its width; this can be represented as p = 2l + 2w. What is the perimeter if l = 12cm and w = 6cm?
18 cm
24 cm
36 cm
48 cm
Perimeter means 'all the way around the outside'
4 .
A cyclist travels at a constant m miles per hour. How far has he travelled in 4 hours?
4m
6m
8m
10m
If m is the number of miles travelled in one hour then the miles covered in 4 hours would be 4 x m or 4m
5 .
What is a 'variable'?
A division sign
A letter or symbol representing a varying quantity
A multiplication sign
An equals sign
We call them variables because their values vary
6 .
If a = 2 and b = 5, which of the following formula is correct?
2a + 6b = 31
3a + b2 = 31
4a + 6b - 2b = 31
5a x 2b = 31
Don't worry, you will NOT normally be presented with formulae that don't add up!
7 .
a2 + b2 = 25. If a = 3 then what is the value of b?
4
5
6
7
If a = 3 then a 2 = 9. 25 - 9 = 16 so b 2 = 16, therefore b must = 4
8 .
What is the plural of formula?
Formulants
Formulas and formulae
Formulents
Formuleses
Either can be used
9 .
Which sign must be contained in a formula?
+
-
x
=
Remember that all formulae contain an equals sign
10 .
If a2 = 6b and a has the value of 6, what is the value of b?
3
6
9
12
In this case you have two variables (a and b) and there is no reason why they cannot both have the same value - in this case 6