## Formulae

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 squarel = length of square Cost of pizzas C = 10x + 5 C = total cost of pizzasx = number of \$10 pizzas orderedThe 5 is the \$5 delivery charge Volume of a prism V =Al V = volume of prismA = area of cross-sectionl = length of prism Distance travelled d = st d = distances = speedt = 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