Sum of an arithmetic or geometric series
Find the sum of the first n terms of an arithmetic or geometric series. Unlimited questions, five difficulty levels, and a full worked solution every time — free.
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A geometric series has first term a1 = 1 and common ratio r = 2. Find the sum of the first 3 terms, S_3.
Answer: 7
See one solved, step by step
A geometric series has first term a1 = 1 and common ratio r = -3. Find the sum of the first 4 terms, S_4.
📘 Worked solution
1Sn = a1(rn - 1)/(r - 1)This is the formula for the sum of a geometric series.
2S4 = 1((−3)4 - 1)/(−3 - 1) = 1(81 - 1)/(−4)Substitute a1 = 1, r = -3, n = 4.
3= −20Simplify to get the total sum.
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