Sum of Series Calculator
Solve sum series problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations.
Formula
Arithmetic: S = n/2(a+l) | Geometric: S = a(1-r^n)/(1-r)
For arithmetic series, S = n/2 times (first + last term). For geometric series, S = a(1 - r^n)/(1 - r) where r is the common ratio. For convergent infinite geometric series (|r| < 1), S = a/(1 - r).
Worked Examples
Example 1: Arithmetic Series Sum
Problem: Find the sum of the first 20 terms of the arithmetic series 5 + 8 + 11 + 14 + ...
Solution: a = 5, d = 3, n = 20\nLast term l = a + (n-1)d = 5 + 19(3) = 62\nS = n/2 x (a + l) = 20/2 x (5 + 62) = 10 x 67 = 670
Result: Sum = 670 | Last term = 62 | Average term = 33.5
Example 2: Infinite Geometric Series
Problem: Find the sum of the infinite geometric series 12 + 6 + 3 + 1.5 + ...
Solution: a = 12, r = 0.5 (|r| < 1, so the series converges)\nFinite sum of 10 terms: S10 = 12(1 - 0.5^10)/(1 - 0.5) = 12(0.999023)/0.5 = 23.977\nInfinite sum: S = a/(1-r) = 12/(1-0.5) = 12/0.5 = 24
Result: Finite sum (10 terms) = 23.977 | Infinite sum = 24
Frequently Asked Questions
What is a mathematical series and how is it different from a sequence?
A sequence is an ordered list of numbers following a pattern, while a series is the sum of the terms in a sequence. For example, 1, 3, 5, 7, 9 is a sequence; 1 + 3 + 5 + 7 + 9 = 25 is the corresponding series. Sequences describe individual terms, while series describe cumulative totals. A finite series sums a specific number of terms, while an infinite series sums infinitely many terms and may or may not converge to a finite value. The distinction matters because a sequence can have well-defined terms without its series converging. Understanding both concepts is fundamental to calculus, analysis, and applied mathematics.
What is an arithmetic series and what is its sum formula?
An arithmetic series is the sum of terms in an arithmetic sequence, where each term differs from the previous by a constant called the common difference (d). The sum of n terms is S = n/2 times (first term + last term), equivalently S = n/2 times (2a + (n-1)d). This formula works because pairing the first term with the last, the second with the second-to-last, and so on produces equal pairs. The famous story of young Gauss summing 1 to 100 exemplifies this: 50 pairs each summing to 101 gives 5050. Arithmetic series appear in financial calculations like total payments on a uniformly increasing schedule and in physics for uniformly accelerated motion.
What is a geometric series and when does it converge?
A geometric series is the sum of terms in a geometric sequence, where each term is multiplied by a constant ratio (r) to get the next term. The finite sum of n terms is S = a(1 - r^n) / (1 - r), where a is the first term. An infinite geometric series converges (has a finite sum) only when the absolute value of r is less than 1, in which case the sum equals a / (1 - r). For example, 1 + 1/2 + 1/4 + 1/8 + ... = 1 / (1 - 0.5) = 2. When the absolute value of r is 1 or greater, the infinite series diverges. Geometric series are fundamental in finance (present value of annuities), physics (bouncing ball total distance), and signal processing.
What is a harmonic series and why is it important?
The harmonic series is the sum 1 + 1/2 + 1/3 + 1/4 + 1/5 + ..., where each term is the reciprocal of a natural number. Despite its terms approaching zero, the harmonic series diverges (grows without bound), though extremely slowly. After summing one million terms, the total is only about 14.39. This counterintuitive result is important because it shows that terms going to zero is necessary but not sufficient for series convergence. The harmonic series appears in probability (coupon collector problem), music theory (overtone frequencies), and computer science (analysis of algorithms). The partial sums of the harmonic series approximate ln(n) + 0.5772 (Euler-Mascheroni constant).
How do you determine if an infinite series converges or diverges?
Several tests help determine convergence. The divergence test says if terms do not approach zero, the series diverges. The ratio test compares consecutive terms: if the limit of |a(n+1)/a(n)| is less than 1, the series converges absolutely. The root test checks if the nth root of |a(n)| has a limit less than 1. The comparison test compares to a known series. The integral test connects series convergence to improper integrals. The alternating series test applies when signs alternate and terms decrease toward zero. For geometric series, convergence requires |r| < 1. No single test works for all series, and choosing the right test is a key skill in calculus courses.
What are power series and how are they used?
A power series is an infinite series of the form sum of a_n times x^n, where x is a variable and a_n are coefficients. Every power series has a radius of convergence R, within which it converges and outside which it diverges. Taylor and Maclaurin series are power series that represent functions. For example, e^x = 1 + x + x^2/2! + x^3/3! + ..., sin(x) = x - x^3/3! + x^5/5! - ..., and 1/(1-x) = 1 + x + x^2 + x^3 + ... for |x| < 1. Power series are essential in physics for approximating solutions to differential equations, in numerical computing for evaluating transcendental functions, and in engineering for analyzing systems near equilibrium.