Ta có
$\underset{x \to 1}{\lim} \dfrac{x^{100} - 1}{x-1} = \underset{x \to 1}{\lim} \dfrac{(x-1)(x^{99} + x^{98} + \cdots + x + 1)}{x-1}$
$= \underset{x \to 1}{\lim} (x^{99} + x^{98} + \cdots + x + 1)$
$= 1 + 1 + \cdots + 1 $ ($100$ số 1)
$= 100$
Vậy
$\underset{x \to 1}{\lim} \dfrac{x^{100} - 1}{x-1} = 100$.