 Site Navigation                            Algebra II Recipe: Arithmetic Series A. Definition B. Finding the sum of the first n terms of an arithmetic sequence
1. Use is the sum of the first n terms.
2. Write a rule for the nth term using the a1 and d values.
3. Find the nth term by substituting the value of n into the rule.
4. Substitute the n, a1, and an values into to find the sum. C. Finding n when Sn is known
1. Substitute the sum for Sn.
2. Substitute the first term for a1.
3. Substitute the rule for an.
4. Solve for n.
• If you get a quadratic equation, solve by factoring or the quadratic formula.
• Choose the positive value.   Examples:   Consider the arithmetic series 4 + 7 + 10 + 13 + 16 + 19 + … Find the sum of the first 30 terms (n = 30).   Consider the arithmetic series 4 + 7 + 10 + 13 + 16 + 19 + … Find n when Sn = 175.

G Redden

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