Changing the subject of a formula
When a formula to find a given quantity is written, the subject of the formula is the letter which is on its own.
Example
A =
 r 2
- Subject A
V = l w h
- Subject V
To change the subject of a formula, we need to re-arrange the parts of the formula to leave a different letter as the subject.

Remember:
To move any term from one side of the equals sign in a formula or equation means that the sign is changed .
If there is a choice of terms to be moved we usually move the add/subtract terms first.
However, we cannot always rely on this rule and we just have to be careful to inspect the equation given.

Make z the subject of the following formula:
1.
Gz = H z =
H
G
G x z = H
2.
8z + T = Y
8z = Y – T z =
Y -
8
T

3.
Lz = Y 2 – h z =
Y 2
L
h
Complete Exercise 4 Questions 1 – 30

Formula with x 2 and negative x terms
When a formula contains an x 2 term that needs to be the subject we must
• Leave the x 2 term on its own first and,
• Then square root the other side to make x the subject.
Example ax 2 = r – t x 2 = r a t x = r a t

When a formula contains a negative x term that is going to be the new subject
• Move the x term to the other side to make it positive,
• Then make it the subject by leaving it on its own
Example h – bx = m h = m + bx h – m = bx h b m = x