∑ is shorthand for 'sum'. Below: starting index. Above: ending index. Inside: the term to sum.
Try this
4
Σ i² from i=1 to n = 1² + 2² + … + n² = 30
Reading a Σ
- Below Σ: the index variable and where it starts (e.g. i = 1).
- Above Σ: where the index stops.
- To the right of Σ: the term — a formula in the index. Plug in each value and add them up.
Handy closed forms
These let you collapse a sum into a single expression — no looping required.
Your turn
Evaluate Σ (2k + 1) for k = 2 to 5.
Try it
∑ᵢ₌₁⁴ i² = ?
1 + 4 + 9 + 16 = 30.