Some terminologies in Wirth's book and the satisfying of deterministic algorithm I am going through Wirth's Textbook on Compiler design . After proceeding though 
some pages ,  I realized I have taken for granted my understanding of some
of the terminologies that have been used . For example ,the following terms :



the examples mentioned in the lines in the screenshot refer to the following set of equations :



Later on while describing the possibility of the conditions of deterministic algorithms being 
satisfied it says for an expresion of the following form :

"term_0 | term_1"

the terms must not feature any common starting symbols .And then it says this results in the exclusion of 
left recusrsion . If we consider the left recursive production :

A = A "a" | "b"

then the requirement is violated "b" is also a start symbol of "A" .

How is "b" a start symbol of "A" ? Basically from the definitions given I understood start symbol means
the symbol for which an equation is deifined first ?

Why does common start symbol disturb the conditions of deterministic algorithm ?
The definition of start symbol that you quote has nothing to do with the definition of start symbol in the context of deterministic grammars. Here is a correct definition:


  A terminal $a$ is a start symbol of a sequential form $\alpha$ if $\alpha \Rightarrow^* aw$ for some $w \in \Sigma^*$.


In words, $a$ is a start symbol of $\alpha$ if $\alpha$ generates some word starting in $a$.

In your example, $Aa$ generates the word $ba$, and so it shares a start symbol with $b$.

Why is sharing a start symbol bad? Suppose you are trying to parse $A$, and are encountering $b$. Which production do you choose? $Aa$ or $b$? It is impossible to tell without looking ahead. The deterministic algorithm considered by Wirth has no lookahead, and so this situation is problematic for it.