New
New
Year 9

# Changing the subject to suit the context

I can evaluate different problems to ascertain when rearranging a formula is beneficial.

New
New
Year 9

# Changing the subject to suit the context

I can evaluate different problems to ascertain when rearranging a formula is beneficial.

## Lesson details

### Key learning points

1. Formulae are used in a variety of places, such as the science lab but also in factories.
2. Rearranging the formula may be beneficial when what you wish to calculate is not the subject.
3. You can compare rearranging the formula to solving for one unknown.

### Common misconception

That the gradient of a line given the equation of the line is always just the coefficient of the x term.

Using graphing software to explore this could help.

### Keywords

• Subject of an equation/formula - 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.

This is a great opportunity to show how these areas of maths link together. Rearranging the equation of a straight line into different forms can be really useful.
Teacher tip

### Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

## Starter quiz

### 6 Questions

Q1.
For the linear equation $$y=5x-7$$, you can call $$y$$ the __________ of the equation.
solution
Q2.
Which of the below are correct rearrangements of the formula $$V=a\times{l}$$ ?
Correct answer: $$V=l\times{a}$$
$$l=a\times{V}$$
Correct answer: $${V\over{l}}=a$$
Correct answer: $${V\over{a}}=l$$
$${l\over{V}}=a$$
Q3.
Make $$y$$ the subject of the equation $$8y-3x=24.$$
$$8y=3x+24$$
$$y={3\over8}x+24$$
Correct answer: $$y={3\over8}x+3$$
$$y=3x+24-7y$$
Correct answer: $$y={{3x+24}\over8}$$
Q4.
Make $$e$$ the subject of $${{c+d}\over{e}}=f.$$
Correct answer: $${{c+d}\over{f}}=e$$
$$f(c+d)=e$$
$${{f}\over{c+d}}=e$$
$$e=cf+df$$
Q5.
Make $$d$$ the subject of $${{c-d}\over{e}}=f.$$
$${{c-f}\over{e}}=d$$
Correct answer: $$c-ef=d$$
$$ef-c=d$$
$$c+ef=d$$
Q6.
Make $$a$$ the subject of $${5\over{a+b}}=c.$$
$${5\over{c+b}}=a$$
$${5c\over{b}}=a$$
$${5c-{b}}=a$$
Correct answer: $${5\over{c}}-b=a$$
$${5\over{c}}+b=a$$

## Exit quiz

### 6 Questions

Q1.
You can divide both sides of the formula $$s\times{t}=d$$ by $$t$$ to make $$s$$ the __________.
divisor
equation
substitution
Q2.
$$s={d\over{t}}$$ can be rearranged as $$s\times{t}=d$$ and $$t={d\over{s}}$$. Which arrangement is the most useful if you know time and speed and wish to calculate distance?
$$s={d\over{t}}$$
Correct answer: $$s\times{t}=d$$
$$t={d\over{s}}$$
Q3.
Which of these arrangements of the formula for area of a circle is most useful if you know the area and wish to find the radius?
$$A={\pi}r^2$$
$${A\over{\pi}}=r^2$$
Correct answer: $$\sqrt{{A\over{\pi}}}=r$$
$${A\over{r^2}}=\pi$$
Q4.
Which rearrangement of the linear equation $$6x+7y=126$$ should you use if you know the $$x$$ coordinate and wish to find the $$y$$ coordinate?
$$6x+7y=126$$
$$x={{126-7y}\over6}$$
Correct answer: $$y={{126-6x}\over7}$$
$$7y=126-6x$$
Q5.
What is the gradient of the straight line with the linear equation $$5y-2x=30$$ ?
$$-2$$
Correct answer: $$2\over5$$
$$2$$
$$5\over2$$
$$5$$
Q6.
The gradient of a straight line is $$-{5\over3}$$ and the $$y$$-intercept is $$(0,20)$$. Give the equation of the line in the form $$ax+by=c.$$
$$5x+3y=20$$
$$3x+5y=20$$
Correct answer: $$5x+3y=60$$
$$5x-3y=60$$
$$3x+5y=60$$