Let fn(x) = (sin nx) / n2. Show that the series fn(x) converges for all values of

Let fn(x) = (sin nx) / n2. Show that the series Σ fn(x) converges for all values of but the series of derivatives Σ f’ n(x) diverges when x = 2nπ, an integer. For what values of does the series Σ f” n(x) converge?