# Doing maths with Susan

Susan, the command line personal assistant, can perform calculations for you.  Simply type your expression followed by the all important equals sign (=).

For example:

`3 + 4 = `

You can set variables using the equals sign as an assignment.

For example:

``` mass = 45.3 ```

Then try:

`force = 0.039`

We can now use our variables in expressions:

`force / mass =`

Susan will keep your variables in memory so they will be available the next time you load her.

You might want to make expressions that are dependent on other variables.  For example, the acceleration is a function of mass and force.  Changing one of these will affect the acceleration.  To do this, use the 'relative assignment' operator (:=).

For example:

`accel := force / mass `

We can now make changes to both 'force' and 'mass' and use the updated 'accel' in other expressions:

` 3 * accel =`

For a full list of built-in mathematical expressions, see the online help - simply type 'help' into Susan to load this up (or press F1).

Still with us?

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