Question: The way I understand the definition of a combination of a set is that, without repetition,for example, the combination of the set $\{1,2\}$ would be $(1,2)$, and not $(1,1)$, $(1,2)$ and $(2,2)$ because I thought I read that each element has to be unique. Is this the case?

Also, 

What is the combination of a set that has only one element? like $\{2\}$ ?

the empty set? or is it $(2,2)$ which would to violate the unique property, if uniqueness actually is a constraint. I'm not sure if it is or not because the web doesn't cover the topic well in a 'non mathematician can understand it' way.

Answer: 
$$\int\frac{u}{\left(a^2+u^2\right)^{\frac{3}{2}}}\space\text{d}u=$$

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Substitute $s=a^2+u^2$ and $\text{d}s=2u\space\text{d}u$:

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$$\frac{1}{2}\int\frac{1}{s^{\frac{3}{2}}}\space\text{d}s=\frac{1}{2}\int s^{-\frac{3}{2}}\space\text{d}s=\frac{1}{2}\cdot-\frac{2}{\sqrt{s}}+\text{C}=-\frac{1}{\sqrt{a^2+u^2}}+\text{C}$$
Let $w=$ some function of $u$ for which $dw = u\,du$.