Formulae

A formula is a rule or equation used to calculate the value of a quantity.

The plural of formula is formulae or formulas.

There are all types of formulae for calculating various quantities in many areas of life.

Examples

Quantity being found
Formula
Variables
Perimeter of a square
P = 4l

P = perimeter of square
l = length of square
Cost of pizzas
C = 10x + 5

C = total cost of pizzas
x = number of $10 pizzas ordered
The 5 is the $5 delivery charge
Volume of a prism
V =Al
 V = volume of prism
A = area of cross-section
l = length of prism
Distance travelled
d = st
 d = distance
s = speed
t = time

The subject of a formula is the term which is often on its own on the left hand side.

e.g. For the cost of pizzas formula above the subject of the formula is C

Substitution into a Formula

To find the value of a quantity in a formula we put given values into a formula in place of the variables (written as letters).

e.g. A = lw is the formula for finding the area of a rectangle.

If      l = 5 cm and w = 7 cm

A = 5 × 7 cm2

A = 35 cm2