Why $\sum_{k=1}^{\infty} \frac{k}{2^k} = 2$? Can you please explain why
$$
\sum_{k=1}^{\infty} \dfrac{k}{2^k} =
\dfrac{1}{2} +\dfrac{ 2}{4} + \dfrac{3}{8}+ \dfrac{4}{16} +\dfrac{5}{32} + \dots =
2
$$
I know $1 + 2 + 3 + ... + n = \dfrac{n(n+1)}{2}$

It's fairly simple. Look at it this way:
$$
\sum_{i=1}^\infty \frac{i}{2^i} = \sum_{i=1}^\infty \frac{\sum_{k=1}^i 1}{2^i} = \sum_{i=1}^\infty \sum_{k=1}^i \frac{1}{2^i}
$$
From here, we just change the order of addition. Rather than adding along $k$, and then $i$, we add along $j=i-k$, and then along $k$. This turns our double sum into
$$
\sum_{i=1}^\infty \frac{i}{2^i} = \sum_{k=1}^\infty \sum_{j=0}^\infty \frac{1}{2^{j+k}} = \sum_{k=1}^\infty \frac{1}{2^k} \sum_{j=0}^\infty \frac{1}{2^j} = 1\cdot 2 = 2
$$
