Sigma notation and series

Sigma notation is compact shorthand for a sum, and reading it correctly is half the battle.

• The number below Σ is the starting value of the counter; the number above is where it stops.
• Substitute the first few values to see the pattern and identify a, d or r.
• The number of terms = (top − bottom) + 1.
• Choose the arithmetic or geometric sum formula once you know the type.

• Sigma (Σ) — notation meaning "the sum of".