Substitution
Replacing each letter in an expression with a number, then working out the answer.
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Analogy
Like filling a blank in a form: the letters are the blanks and the numbers are what you write in.
Example
2 x 3^2 - 5= 2 x 9 - 5= 18 - 5= 13
In practice
A taxi fare formula such as 3 + 2m gives a price once you put in the miles.
Practice
Flashcards
- What is 2x^2 - 5 when x = 3?13. Square first (9), then 2 x 9 = 18, then take away 5.
Questions people ask
Why do you square before multiplying in 2x^2?
Powers only apply to the thing directly next to them. In 2x^2 only the x is squared, not the 2. With x = 3, you work out 3^2 = 9 first, then multiply by 2 to get 18. This follows BIDMAS.
What happens when you substitute a negative number?
Put the negative number in brackets so the sign is not lost. For x = -2 in x^2, write (-2)^2 = 4, which is positive. Without brackets you might wrongly get -4. Brackets keep the whole number squared. Calculators need the brackets typed in too. Forgetting them is one of the most common ways to lose marks in substitution questions.