Question
Let $a_1, a_2, ....,a_n$ be fixed real numbers and define a function $f(x) = (x - a_1)(x - a_2)... (x - a_n). $ What is & $\mathop {\lim }\limits_{x \to {a_1}} f(x)$? For some $a \ne {a_1},\;{a_2}...{a_n},$ compute $\mathop {\lim }\limits_{x \to {a}} f(x)$

Answer

Here $f(x) = (x – a_1) (x – a_2)… (x – a_n)$
Now $\mathop {\lim }\limits_{x \to {a_1}} f(x) = \mathop {\lim }\limits_{x \to {a_1}} (x - {a_1})(x - {a_2})...(x - {a_n})$
$= (a_1 – a_1) (a_1 – a_2) … (a_1 – a_n)$
$= 0 \times (a_1 – a_2) … (a_1 – a_n) = 0.$
Also $\mathop {\lim }\limits_{x \to a} f(x) = \mathop {\lim }\limits_{x \to a} (x - {a_1})(x - {a_2})...(x - {a_n})$
$= (a – a_1) (a – a_2) … (a – a_n)$

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