Confusion with Summation notation I need to compute the value of this:
$$\frac{1}{3}\left(\sum_{n=1}^{3} \frac{z_{n}}{f} + N(2,2)\right)$$
the $N$ is a gaussian noise with mean=2 and standard deviation=2. 
The question:
is this equivalent to:
$$
\frac{1}{3}\left(\frac{z_{1}}{f} + N(2,2)+\frac{z_{2}}{f} + N(2,2)+\frac{z_{3}}{f} + N(2,2)\right)
$$
or 
$$
\frac{1}{3}\left(\left(\frac{z_{1}}{f} +\frac{z_{2}}{f} +\frac{z_{3}}{f}\right) + N(2,2)\right)
$$

It could be either one. It is more likely to be the latter; at least I myself would interpret it as
$$\frac{1}{3}\left(\sum_{n=1}^{3} \frac{z_{n}}{f} + N(2,2)\right)=\frac{1}{3}\left(\left(\frac{z_{1}}{f} +\frac{z_{2}}{f} +\frac{z_{3}}{f}\right) + N(2,2)\right),$$
because the former would more likely be written with extra braces, i.e.
$$\frac{1}{3}\left(\frac{z_{1}}{f} + N(2,2)+\frac{z_{2}}{f} + N(2,2)+\frac{z_{3}}{f} + N(2,2)\right)=\frac{1}{3}\left(\sum_{n=1}^{3}\left(\frac{z_{n}}{f} + N(2,2)\right)\right),$$
but not everyone is as generous with the braces, and some may omit them to avoid clutter.
Your best chances at interpreting the expression properly is to inform with whomever gave it to you.
