Partial Sum Formula – Definition, Applications, and Examples

Partial Sum Formula Definition Application and

In this article, we will unravel the intricacies of the partial sum formula, demonstrating its immense utility and showcasing the beauty of mathematical abstraction and simplification.

Definition of Partial Sum Formula

The partial sum formula pertains to the summation of a subset of terms from a sequence or series. In the context of sequences and series, a “partial sum” is the sum of the first n terms of the sequence. Mathematically, if $a_i$ represents the i-th term of a sequence, then the n-th partial sum, denoted $S_n$ , can be defined as:

$S_n = a_1 + a_2 + a_3 + … + a_n$

The “partial sum formula,” in particular, refers to an explicit formula that allows one to compute $S_n$ directly without having to sum each of the terms individually. The exact form of this formula will vary depending on the specifics of the sequence or series in question.

For instance, for an arithmetic sequence where the difference between consecutive terms is constant, the partial sum formula is:

$S_n = \frac{n}{2} (2 a_1 + (n – 1) d)$


$a_1$ is the first term of the sequence. d is the common difference between terms. n is the number of terms being summed. For other types of sequences or series, the partial sum formula will be different.


The partial sum formula provides an efficient way to compute the sum of the first n terms of a sequence. The exact form of the formula depends on the type of sequence or series. We’ll delve into the properties of the partial sum formula as they pertain to some of the most common sequences and series.

Arithmetic Sequences

An arithmetic sequence is a sequence in which the difference between consecutive terms is constant, denoted by d.

Partial Sum Formula

$S_n = \frac{n}{2} (2a_1 + (n – 1) d)$


The sum is the average of the first and last term multiplied by the number of terms, n. The sum is linearly dependent on n, meaning the total increases linearly as we sum more terms.

Geometric Sequences

A geometric sequence is a sequence in which each term after the first is found by multiplying the previous term by a fixed, non-zero number called the ratio, r.

Partial Sum Formula (for r ≠ 1)

$S_n = \frac{a_1}{1 – r^n} \cdot \frac{1 – r}{1 – r}$


If |r| < 1, as n grows, r^n approaches zero, making the formula tend toward a limit, i.e., the sum of an infinite geometric series. If r > 1, the sum grows exponentially with n.


Some sequences have infinite terms. The partial sum of these sequences can either converge (approach a finite value) or diverge (grow without bound). The behavior of the partial sum formula can help determine if a series converges or diverges.


Given two sequences with known partial sum formulas, the partial sum of their linear combination (i.e., adding them together after multiplying by constants) is the linear combination of their partial sums. In mathematical terms:

If $S_n$ is the partial sum of sequence $a_n$ and $T_n$ is the partial sum of sequence $b_n$, then the partial sum of c $\cdot a_n + d \cdot b_n$ is $ c \cdot S_n + d \cdot T_n$ , where c and d are constants.


The order in which terms are added does not change the partial sum. So, if you reverse the order of a sequence, the partial sum remains unchanged.

Shift Property

Shifting the sequence (i.e., adding or removing terms at the beginning) changes the partial sum by adding or subtracting the removed or added terms. This property underlines the importance of the starting index in determining the partial sum.


Example 1

Find the sum of the first 100 natural numbers.


Using the formula, $S_n = \frac{n}{2} (2 a_1 + (n – 1) d)$:

$a_1$ = 1 (first term)

$a_n$ = 100 (last term)

n = 100

$S_{100} = 2 \cdot 100 \cdot \left(1 + \frac{100 – 1}{2}\right)$ = 5050

Example 2

Find the sum of the first 50 even numbers.


Using the formula, $S_n = \frac{n}{2} (2 a_1 + (n – 1) d)$:

$a_1$ = 2

d = 2 (common difference)

n = 50

$S_{50} = 2 \cdot 50 \cdot \left(2 + \frac{50 – 1}{2} \cdot 2\right)$ = 2550

Example 3

Find the sum of the first four terms of the series that starts with 3 and has a common ratio of 2.


Using the formula, $S_n = \frac{a_1}{1 – r^n} \cdot \frac{1 – r}{1 – r}$:

$a_1$ = 3

r = 2

n = 4

$S_4 = 3 \cdot \frac{1}{1 – 2^4} \cdot \frac{1 – 2}{1 – 2}$ = 45

Example 4

Find the sum of an infinite geometric series that starts with 1 and has a common ratio of 0.5.


The formula for an infinite geometric series is $S_\infty = \frac{a_1}{1 – r}$:

$a_1$ = 1

r = 0.5

$S_\infty = \frac{1}{1 – 0.5}$ = 2

Example 5

Find the sum of the arithmetic series: 5, 8, 11, … , 56.


$a_1$ = 5

d = 3

n = 18

$S_{18} = 18 \cdot \frac{1}{2}$ (5 + 56) = 549

Example 6

Find the sum of the arithmetic series that has 15 terms, starts with 4, and ends with 64.


Using the formula, $S_n = \frac{n}{2} (2 a_1 + (n – 1) d)$:

$a_1$ = 4

$a_n$ = 64

n = 15

$S_{15} = 15 \cdot \frac{1}{2}$ (4 + 64) = 510


The partial sum formula is pivotal in various fields, underscoring its universal importance. Here’s how it impacts diverse domains:

  • Physics

    • Harmonic Series: In the study of oscillations and waves, the behavior of overtones and harmonics can be described using series, with the partial sum formula helping in predicting the behavior of a limited number of harmonics.
    • Quantum Mechanics: The probability amplitudes of certain states can be represented as series, and the convergence of these series is crucial for physical interpretations.
  • Engineering

    • Signal Processing: Fourier series, which represents a function or a signal in terms of a sum of sinusoids, employs the concept of partial sums. Engineers may use partial sums of the series to approximate signals.
    • Control Theory: The behavior of systems, especially those represented by differential equations, can be described using series. The stability and behavior of these systems can be analyzed using the convergence of these series.
  • Computer Science

    • Algorithms: The running time of some algorithms can be represented as a series, especially when discussing average or worst-case scenarios.
    • Graphics: In rendering, particularly with ray tracing, series are sometimes used to compute light interactions. Partial sums can offer approximations when complete computations are too intensive.
  • Economics and Finance

    • Forecasting: Future revenues or economic indicators can be modeled using series, especially when considering recurring factors.
    • Investment: Compound interest and investment growth can be modeled with geometric series, where the partial sum formula can predict the future value of an investment over a given number of periods.
  • Biology and Medicine

    • Population Growth: Geometric or exponential sequences and series can model population growth. The partial sum formula can predict the number of individuals in future generations.
    • Pharmacology: The effects of recurring drug doses can be represented using series, especially if the drug’s effect diminishes over time.
  • Mathematics

    • Number Theory: The study of integers and more abstract constructs often employs series and their properties.
    • Calculus: Series solutions to differential equations or representations of functions as Taylor or Maclaurin series are common applications of the partial sum formula.
  • Environmental Science

    • Climate Modeling: Series, notably Fourier series, is sometimes used in modeling temperature changes or other cyclical phenomena.