$$\sum_{n=1}^{\infty} n^{3}x^{n-1}$$
The answer is supposed to be: (according to wolfram alpha)
$$ \frac{x^2+4x+1}{(x-1)^4} $$
I have only learned to to this for simpler geometric sums.
How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...
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