The partial sum $s_n=\frac{1}{4}(3^n\sin\frac{a}{3^n}-\sin\ a)$
$=>S=\lim_{n\to\infty} s_n=\frac{1}{4}\lim_{n\to\infty}\left[a\frac{\sin\frac{a}{3^n}}{\frac{a}{3^n}}-\sin \ a\right]\ \ (*)$
My question is about how that change occurred in (*). How it went from $3^n\sin\frac{a}{3^n}$ to $a\frac{\sin\frac{a}{3^n}}{\frac{a}{3^n}}$. (I know how to evaluate that limit)