igcse ppt

2. Changing the Subject
• The topic we will learn about today is
‘changing the subject’
• This is a very useful skill which you can
use with any formulae you may need to
use
• It involves formulae which have more
than one ‘unknown’ value

3. Changing the Subject
• In the formula;
y = 2x
y is known as the ‘subject’ of the formula
This is because you are being given y ‘in terms of’ x
It is possible to re-arrange the formula to give x ‘in
terms of y’
This is called ‘changing the subject’

4. Changing the Subject
y = 2x
To make x ‘the subject’, you need to re-
arrange the formula to get x on its own.
y = 2x
y/2 = x
÷ 2 ÷ 2

5. Changing the Subject
a = 3b - 1
To make b ‘the subject’, you need to re-arrange the
formula to get b on its own.
a = 3b - 1
= 3b
a + 1
a + 1 = b
3
+ 1 to both
sides
Divide both
sides by 3

6. Changing the Subject
m = 7n + 5
To make n ‘the subject’, you need to re-arrange the
formula to get n on its own.
m = 7n + 5
= 7n
m - 5
m - 5 = n
7
- 5 from
both sides
Divide both
sides by 7

7. Changing the Subject
s = 3t + 5u
To make t ‘the subject’, you need to re-arrange the
formula to get t on its own.
s = 3t + 5u
= 3t
s - 5u
s - 5u = t
3
- 5u from
both sides
Divide both
sides by 3

8. Changing the Subject
s = 3t + 5u
To make u ‘the subject’, you need to re-arrange the
formula to get u on its own.
s = 3t + 5u
= 5u
s - 3t
s - 3t = u
5
- 3t from
both sides
Divide both
sides by 5

9. Changing the Subject
s = 3t + 5u
s - 3t
=
u
5
s - 5u
=
t
3
So we have got 3
formulae, all from 1
starting one!

10. Changing the Subject
a = 3(b + c)
To make c ‘the subject’, you need to re-arrange the
formula to get c on its own.
a = 3(b + c)
= b + c
a
3
Divide by 3
Subtract b
= c
a
3
- b

11. Changing the Subject
a = b/3 + 5
To make b ‘the subject’, you need to re-arrange the
formula to get b on its own.
a = b/3 + 5
Subtract 5
Multiply by 3
a - 5 = b/3
3(a - 5) = b

12. Plenary
a - 4
c/4
4k + 3

13. Plenary
w
=
5(2 + t)
-10 from
both sides
Expand the
bracket
10 + 5t w
=
5t w - 10
=
t w - 10
=
5
Divide both
sides by 5

14. Plenary
w
=
5(2 + t)
-2 from
both sides
Divide both
sides by 5
w
=
2 + t
5
w
=
t
5
- 2

15. Summary
• We have learnt how to change the
subject of a formula
• We have seen how to do this with 2 or
more ‘unknowns’
• We have also looked at some example
SAT questions