Click here to purchase the entire book in PDF format. Functions When you have an equation, you are saying that something is equal to something else. For example, look at Equation 1.68. $$y = 2x$$ (2.68) This says that the value of $y$ is calculated by getting a value for $x$ and multiplying that by 2 (I really hope that this is not coming as a surprise...). A function is basically the busy side of an equation. Frequently, you will see books talking about a function $f(x)$ which just means that you do something to $x$ to come up with something else. For example, in Equation 1.68, the function $f(x)$ (when you read this out loud, you say ``$f$ of $x$'') is $2x$. Of course, this can be much more complicated, but it can still just be thought of as a function - do some math using $x$ and you'll get your answer.