Sylvain G. writes: I just want to report that I was not able to add the following line with mathjax on my blogger:

Trying this: $$\begin{align*}\frac{x^n- a^n}{x-a} &= \frac{a^n\left((\frac{x}{a})^n - 1\right)}{a\left(\frac{x}{a}-1\right)}\\ &= a^{n-1}\left( \left(\frac{x}{a}\right)^{n-1} + \left(\frac{x}{a}\right)^{n-2} +\ldots + \frac{x}{a} + 1 \right) \\ &= x^{n-1} + x^{n-2}a + +\ldots + xa^{n-2} + a^{n-1}. \end{align*}$$