Equations and Formulas

What is an Equation?

An equation says that two things are equal. It will have an equals sign "=" like this:

x

+

2

=

6

That equations says: what is on the left (x + 2) is equal to what is on the right (6)

So an equation is like a statement "this equals that"

What is a Formula?

A formula is a special type of equation that shows the relationship between different variables.

(A variable is a symbol like x or V that stands in for a number we don't know yet).

Example: The formula for finding the volume of a box is:

V = hwl

V stands for volume, h for height, w for width, and l for length.

 When h=4, w=5, and l=10, then V = 4 × 5 × 10 = 200

A formula will have more than one variable.

These are all equations, but only some are formulas:

x = 2y - 7	Formula (relating x and y)
a2 + b2 = c2	Formula (relating a, b and c)
x/2 + 7 = 0	Not a Formula (just an equation)

Without the Equals

Sometimes a formula is written without the "=":

Example: The formula for the volume of a box is:

hwl

But in a way the "=" is still there, because you could write V = hwl if you wanted to.

Subject of a Formula

The "subject" of a formula is the single variable (usually on the left of the "=") that everything else is equal to.

Example: in the formula

s = ut + ½ at²

"s" is the subject of the formula

Changing the Subject

One of the very powerful things that Algebra can do is to "rearrange" a formula so that another variable is the subject.

Rearrange the volume of a box formula (V = hwl) so that the width is the subject:

Start with:	V = hwl
divide both sides by h:	V / h = wl
divide both sides by l:	V / hl = w
swap sides:	w = V / hl

So now if you have a box with a length of 2m, a height of 2m and a volume of 12m³, you can calculate its width:

w = V / hl

w = 12m³ / (2m×2m) = 12/4 = 3m