\(\displaystyle\sum_{n=1}^{10} \mbox{ } (2n + 1) = (2 \cdot 1 + 1) + (2 \cdot 2 + 1) + \mbox{ }... \mbox{ } + (2 \cdot 10 + 1) = 120\),
for example, when n = 2 (the number under the summary sign), then start with 2 till the number over the summary sign. If you don't get it, e.g.
\(\displaystyle\sum_{n=2}^{5} \mbox{ } n + 1\) you see n = 2, so (2 + 1), then comes n = 3, so (2 + 1) + (3 + 1) + ... (5 + 1) = 18,
so \(\displaystyle\sum_{n=2}^{5} \mbox{ } n + 1 = 18\) .
Feel free to ask if you still don't get it.
