Un summa es le resultato de un addition. Anque le resultato de un serie es un summa. Pro additiones longe e regular, on usa le symbolo Σ {\displaystyle \Sigma } , lege: sigma.
Per exemplo, ∑ i = 1 6 i = 1 + 2 + 3 + 4 + 5 + 6 = 21 {\displaystyle \sum _{i=1}^{6}i=1+2+3+4+5+6=21} . In general, pro omne n ∈ N {\displaystyle n\in \mathbb {N} } , vale ∑ i = 1 n i = n ( n + 1 ) 2 {\displaystyle \sum _{i=1}^{n}i={\frac {n(n+1)}{2}}} . In le exemplo: ∑ i = 1 6 i = 6 ( 6 + 1 ) 2 = 6 ⋅ 7 2 = 42 2 = 21 {\displaystyle \sum _{i=1}^{6}i={\frac {6(6+1)}{2}}={\frac {6\cdot 7}{2}}={\frac {42}{2}}=21} . Un altere exemplo: ∑ i = 1 100 i = 100 ( 100 + 1 ) 2 = 5050 {\displaystyle \sum _{i=1}^{100}i={\frac {100(100+1)}{2}}=5050} .
