Show how a sentence can be produced from a grammar (Dragon book 2.1) In the Dragon book (Aho, Sethi, Ullmann) there is one exercise I don't get.


  Chapter 2, Exercies: 2.1
  Given the context-free grammar
  $$S \to S S + \mid S S * \mid a$$
  Task: Show how the signs aa+a* can be produced from this grammar.


I understand the grammar, $S$ can have the form $S S +$… and so on. But I don't know what I'm supposed to do in this task. Sadly I can't find any solutions on the web.
$$\begin{align*}
S \Rightarrow &amp; SS* \\
  \Rightarrow &amp; SS+S* &amp; \text{(leftmost derivation )} \\
  \Rightarrow &amp; aS+S* \\
  \Rightarrow &amp; aa+S* \\
  \Rightarrow &amp; aa+a* \\
\end{align*}$$