The symbol, ∑ is a summation sign that drills
us to sum the elements of a sequence. The variable of summation is represented
by an index that is placed under the summation sign and is often embodied by i.
The index is always equal to 1 and adopt values beginning with the value on the
right hand side of the equation and finishing it with the value over head the
summation sign. So in the expression ∑4i = 0(5)(1/3)i will be the same as: