How to solve
The sum of an arithmetic series is the average of first and last terms times how many terms: Sₙ = n(a₁ + aₙ)/2.
Steps
- Find the last term aₙ = a₁ + (n − 1)d.
- Average it with a₁.
- Multiply by n.
Example
Sum of the first 10 terms of 3, 7, 11, …
- a₁₀ = 3 + 9·4 = 39.
- S₁₀ = 10(3 + 39)/2 = 210.
Answer: 210
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