Working with formulas > Using formulas
1234Using formulas

## Theory

A formula is a sentence containing variables. Formulas often describe a relation between those variables, but not always.
Formulas often look like an equation, i.e. a sentence with an equals sign.
If a formula describes a relation between two variables, you can draw a corresponding graph. First, you make a table. Next you draw the points in a coordinate system and connect them.

In practice, formulas often describe relationships between quantities. Those quantities are represented by a variable, a letter which corresponds to the name of the quantitiy used.
A quantity is measured in units.

• The formula $A={z}^{2}$ is an equation defining the relation between the variables $A$ and $z$. You can make the corresponding table and plot the corresponding graph.

• The formula $2t+40=300$ gives information about the unknown $t$. The solution to this equation is $t=130$, for $2\cdot 130+40=300$.

• The formula ${\left(x+3\right)}^{2}={x}^{2}+6x+9$ is a calculation rule and is valid for all $x$.