[tex]\bf \begin{array}{ccll}
n&f(n)\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
1&540\\
2&\stackrel{540-40}{500}\\\\
3&\stackrel{500-40}{460}\\\\
4&\stackrel{460-40}{420}\\\\
5&\stackrel{420-40}{380}
\end{array}[/tex]
notice, the next term is obtained by subtracting 40 from the "current" one, meaning, is an arithmetic sequence, with a "common difference" of -40.
