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Rearranging Formulas (Edexcel GCSE Maths: Foundation): Flashcards

Exam code: 1MA1

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FrontRearranging Formulas
Define the subject of a formula.

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Cards in this collection (6)

Define the subject of a formula.
The subject is the letter that appears on its own on one side of the formula, so in $y = m x + c$ the subject is $y$ .
Making a different letter the subject is called changing the subject.
When you make $x$ the subject of a formula, what kind of answer do you end up with?
You end up with an expression written in terms of the other letters, not a number.
A formula contains more than one unknown quantity, so there is no single value of $x$ waiting to be found.
Complete the rearrangement of $4 m + 5 x = 3$ to make $x$ the subject:
$x = \frac{______-______}{5}$
The completed rearrangement is:
$x = \frac{3 - 4 m}{5}$
Subtracting $4 m$ from both sides gives $5 x = 3 - 4 m$ , and the numerator keeps that order: it is $3 - 4 m$ , not $4 m - 3$ .
To make $x$ the subject of $(1 + k) x = y$ , do you need to expand the bracket?
You do not need to expand: the $x$ is outside the bracket, so divide both sides by the whole bracket to get $x = \frac{y}{1 + k}$ .
Expand only when the letter you want is inside the bracket.
True or False?
$x = \frac{y - 3}{3}$ and $x = \frac{y}{3} - 1$ are both correct ways of making $x$ the subject of $3 (1 + x) = y$ .
True.
Both are correct, because $\frac{y - 3}{3}$ and $\frac{y}{3} - 1$ have the same value for every value of $y$ .
A rearranged formula can often be written in more than one correct form, so two answers that look different can both be right.
Rearranging a formula gives $x = \frac{y - 3}{- 2}$ . If the minus sign is moved to the top, why must the numerator be written as $- (y - 3)$ ?
The minus sign applies to the whole numerator, and the brackets are what show that.
Without them the expression would read $- y - 3$ , which leaves the $3$ with the wrong sign, whereas $- (y - 3)$ correctly gives $- y + 3$ .

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