now I'm solving some past job interview problems.I have met a embarrassing question about compiler.the question is :Consider the following grammar, with start symbol $E$:\begin{align*}E &\rightarrow E \ast E\\ &\quad \mid E ~/~ E\\ &\quad \mid E + E\\ &\quad \mid E - E\\ &\quad \mid (E)\\ &\quad \mid a \mid b \mid c \mid \ldots \mid x \mid y \mid z\end{align*}The following strings are legal derivations from this grammar:$a \ast b + c$$( a - b ) \ast c$$a ~/~ ( b  c)$Which of the above are rightmost sentential forms?when I was undergraduate student I learned that rightmost(leftmost) sentential form is result of rightmost(leftmost) derivation.so, I think if $1,2,3$ are legal derivations form above grammar then absolutely all of $1,2,3$ are rightmost sentential form.if I'am correct, above question is very worthless.is there any counter example that is not rightmost sentential form of grammar $G$ although it is legal derivation from $G$?ORis there another definition about rightmost sentential form corresponding to above question? I have googled rightmost sentential form. but there are few results and most of them is the definition that I've already known.
What is rightmost sentential forms?
formal grammars;compilers