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# Equations

An equation expresses a relationship between (usually) two or more variables. For an equation to be a function there is only one output for each input.

#### For example:

In Economics, supply and demand are each represented by an equation.

Each equation has 2 variables: price ($P$) in dollars and quantity demanded ($Q$).

Demand Curve: $P=16-2Q$

Supply Curve: $P=4+Q$

A value for $Q$ can be substituted into the equation.

When $Q=4$ the demand curve tells us the price $P=16-2 \times 4=8$

Review the use of  order of operations as well as  positive and negative numbers to ensure you evaluate the equation correctly.

## Function notation:

For example:

Let $B(t)$ denote the bank balance after $t$ days.

$B(t)= 100 + 10t$

This is the function where the value of the bank balance is dependent on $t$, the number of days.

$B(3) = 100 + 10 \times3 = 130$    The value of the bank balance is $130$ after $3$ days.

## Further information

• Press the Printer Friendly button at the bottom left-hand corner to download a printable handout
• RMIT University has online  videos to review the use of  equations.