What does this mean...
(2)
(n)
where 2 an 2 are actually all n one big bracket. There is a "Sum of" sign before it.
Thank you!
no, you write the upper and lower limits of a sum on the summation sign itself, like, for example, you would write $\displaystyle \sum_{n = 1}^N a_n$ 1 and N are the lower and upper limits respectively of the sum
Now, the notation $\displaystyle {n \choose k} = C(n,k) = _nC_k = C_k^n$ means the combination as in the combinatoric formula. it is defined as follows:
$\displaystyle {n \choose k} = \frac {n!}{k!(n - k)!}$
see here
so, for example, $\displaystyle \sum_{n = 0}^2 {2 \choose n} = {2 \choose 0} + {2 \choose 1} + {2 \choose 2} = 1 + 2 + 1 = 4$ .......here the upper and lower limits are 2 and 0 respectively