A formula is a general rule or relation, written as an algebraic equation.
There are all types of formulae for calculating various quantities in many subject areas.
Substitution
Substitution involves putting given values into a formula in place of the variables (written as letters).
Example 1

Example 2

is the formula for the area of a trapezium.
If a = 5 cm, b = 7 cm and h = 10 cm A = 60 cm^{2} 
V = IR is a formula used in physics, where V = voltage, I = current and R = resistance. If I = 10 amps and R = 5 ohms V = 10 × 5 V = 50 volts

Changing the Subject
The subject of a formula is the variable or letter that stands alone.
e.g.
A is the subject 

x + y + z = V 
V is the subject 
To make a different variable the subject of a formula, the usual algebraic techniques can be used.
Example 1

Example 2

To make b the subject of 
v^{2} = u^{2} − 2fs is a formula used in physics. To make u the subject 
Rearranging Rational Functions
Often rational expressions such as have to be rearranged to make x the subject.
Rearranging formulae with Squares and Square roots
To rearrange formulae involving squares the square root often needs taking.
To rearrange formulae involving square roots both sides of the formulae need squaring.
Example involving square roots

Example involving squares
