1. Changing the subject of the formula
a + b = c
c is the
subject
Rearrange the formula to make b the subject so that
The
method
is the
same as
solving
equations
b = ……...
- a from both sides
a + b = c -a -a
b = c - a

2. Make b the subject a = bx + c
rearrange so that b is on the right
bx + c = a
bx + c = a
- c from both sides
- c
- c
bx = a - c
÷ both sides by x
÷ x
÷ x
bx = a - c
b = a – c x

3. Make s the subject n = m - 3s
add 3s to both sides
n + 3s = m
- n from both sides
3s = m - n
s = m - n
divide both sides by 3 3

4. Make r the subject reverse to get r on left multiply both sides by s
p = q + r s
q + r = p s
reverse to get r on left
multiply both sides by s
q + r = sp
r = sp – q
- q from both sides

5. Make b the subject reverse to get b on left multiply out the bracket
s = 3(a + b)
reverse to get b on left
3(a + b) = s
multiply out the bracket
3a + 3b = s
- 3a from both sides
3b = s - 3a
3b = s - 3a 3
divide both sides by 3

6. The area of a trapezium is
Find h (height) when
A = 40 a = 10 b = 6
A = h(a + b) 2
Make h the subject then substitute in the values
How??
1st rearrange the formula 2
h(a + b) = A 2
h(a + b) = A x2
h(a + b) = 2A
h(a + b) = 2A
h = 2 Aa
a + b
divide by (a + b)
substitute in the values A = 40, a = 10, b = 6 80 16
h = 2 x 40 =
10 + 6
= 5

7. Turn to page 117 Complete exercise 18.1A
Then exercise 18.1B on page 118