s i = s o m e t h i n g i a n o t h e r t h i n g {\displaystyle s_{i}={\frac {something_{i}}{anotherthing}}}
s s s = ∑ i = 1 n s i {\displaystyle sss=\sum _{i=1}^{n}s_{i}}
or
s s s = ∑ i ∈ A s i {\displaystyle sss=\sum _{i\in A}s_{i}}
