$$f(x) = \lim_{n\to \infty} \sum_{r=1}^n 3^{r-1}\sin^3(x/(3^r)) $$ I tried using the formula relating $\sin(3x)$ to $\sin^3(x)$ but got later stuck with a similar series who's sum I didn't know how to calculate
$$f(x) = \lim_{n\to \infty} \sum_{r=1}^n 3^{r-1}\sin^3(x/(3^r)) $$
I tried using the formula relating $\sin(3x)$ to $\sin^3(x)$ but got later stuck with a similar series who's sum I didn't know how to calculate
