Checking and securing understanding of changing the subject | Maths

Starter quiz

If the side of an equilateral triangle is $3n+1$ , write an expression for the perimeter.

'3(3n+1)' ✓
If the side of a regular pentagon is $14y - a$ write an expression for the perimeter.

'5(14y - a)' ✓
Which of the following is an expression for the area of a square with side length $3x$

$3x^2$

$(3x^2)$

$(3x)^2$ ✓
What is the value of $x$ for $5(x + 7) = 30$ ?

'-1' ✓
If $x$ is a positive integer, what is the value of $x$ for $x^2 - 4 = 0$ ?

'2' ✓
What is the value of $x$ for $2x - 3 = 7$ ?

'5' ✓

Exit quiz

Make $x$ the subject of $3y - x = 15$

$x = 3y - 15$ ✓

$x = 15 - 3y$

$x = 3y + 15$

$x = -3y - 15$
Make $y$ the subject of $4x + 2y = 8$

$y = 2x + 4$

$y = -2x - 4$

$y = 4 - 2x$ ✓

$y = 2 - 4x$
Make $z$ the subject of $\frac{z}{2} + 3x = 7$

$z = 7 - 3x$

$z = 4 - 3x$

$z = 14 - 3x$

$z = 14 - 6x$ ✓
Make $a$ the subject of $b = 3(a - 4) + 2$

$a = \frac{b + 2}{3} - 4$

$a = \frac{b - 2}{3} + 4$ ✓

$a = \frac{b - 2}{3} - 4$

$a = \frac{b + 6}{3}$
Make $m$ the subject of $\frac{4m + 5}{3} = n$

$m = 3n + 5$

$m = \frac{3n + 5}{4}$

$m = \frac{3n - 5}{4}$ ✓

$m = \frac{n - 5}{3}$
Make $p$ the subject of $2p + \frac{q}{3} = 4$

$p = \frac{8 - q}{6}$

$p = 2 - \frac{q}{3}$

$p = \frac{4 - q}{3}$

$p = 2 - \frac{q}{6}$ ✓

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Lesson Details

Key learning points

Rearranging the formula may be beneficial when what you wish to calculate is not the subject.
You can compare rearranging the formula to solving for one unknown.
Instead of finding a value though, you will find an expression that the variable is equal to.

Common misconception

When rearranging multiplicative relationships, the values just 'swap'

Pupils should understand how to manipulate multiplicative relationships properly instead of relying on a 'trick'. This will be useful in other subject areas as well.

Keywords

Subject of an equation - The subject of an equation/a formula is a variable that is expressed in terms of other variables. It should have an exponent of 1 and a coefficient of 1.