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IB DP Maths AA SL Study Notes

1.2.2 Advanced Sigma Notation

Summation Properties

Distributive Property

The distributive property allows a sum to be separated into two or more separate sums, which can be particularly useful when dealing with expressions that can be broken down into simpler parts. For a more detailed introduction, you can refer to our introduction to sigma notation.

  • Sum (from n=a to b) [f(n) + g(n)] = Sum (from n=a to b) f(n) + Sum (from n=a to b) g(n)

Factor Rule

The factor rule enables us to take a constant multiplier outside of the sigma notation, simplifying the expression inside the sum.

  • Sum (from n=a to b) cf(n) = c Sum (from n=a to b) f(n)

Power Rule

The power rule allows for the simplification of sums involving powers, which can be particularly useful when dealing with polynomial expressions. This concept can be further explored in the power rule section.

  • Sum (from n=a to b) [f(n)]k = [Sum (from n=a to b) f(n)]k

Example 1: Distributive Property and Factor Rule

Consider the sum S = Sum (from n=1 to 5) [2n + 3n2]. Using the distributive property and factor rule, we can write:

S = Sum (from n=1 to 5) 2n + Sum (from n=1 to 5) 3n2 S = 2Sum (from n=1 to 5) n + 3Sum (from n=1 to 5) n2

Calculating each sum separately, we get:

S = 2(1 + 2 + 3 + 4 + 5) + 3(1 + 4 + 9 + 16 + 25) S = 2(15) + 3(55) S = 30 + 165 S = 195

Series Calculations

Telescoping Series

A telescoping series is a series where each term cancels out with a preceding or succeeding term. When summed, many terms conveniently cancel out, making the calculation straightforward. This concept is fundamental in various areas, such as the basics of rational functions.

Example 2: Telescoping Series

Consider the series S = Sum (from n=1 to ∞) [1/n - 1/(n+1)]. Notice that each term will cancel out with the next, leaving only the first term, 1, as n approaches infinity. Thus, S = 1.

Nested Summations

Nested summations involve having one or more sigma notations within another. These can be evaluated by working from the innermost sum to the outermost. For more advanced techniques in summations, explore the topic of proof by mathematical induction.

Example 3: Nested Summations

Evaluate the sum S = Sum (from i=1 to 3) Sum (from j=1 to 2) ij.

To evaluate nested summations, we first evaluate the inner sum for each value of the outer sum:

S = Sum (from i=1 to 3) [i(1) + i(2)] S = Sum (from i=1 to 3) [i + 2i] S = Sum (from i=1 to 3) 3i

Now, we evaluate the outer sum:

S = 3(1) + 3(2) + 3(3) S = 3 + 6 + 9 S = 18

Example Questions in Study Notes

Example 4: Sum of Squares

Evaluate the sum S = Sum (from n=2 to 5) n2.

To find the sum, substitute each value of n from 2 to 5 and add the resulting terms:

S = 22 + 32 + 42 + 52 S = 4 + 9 + 16 + 25 S = 54

Example 5: Sum of Linear Expression

Evaluate the sum S = Sum (from n=1 to 4) [2n + 3]. This example closely relates to the basics of binomial expansion.

To find the sum, substitute each value of n from 1 to 4 and add the resulting terms:

S = [2(1) + 3] + [2(2) + 3] + [2(3) + 3] + [2(4) + 3] S = 5 + 7 + 9 + 11 S = 32

In these notes, we have delved into advanced sigma notation, exploring its properties, series calculations, and nested summations. Through the examples provided, students can observe the application of sigma notation in various contexts, enhancing their problem-solving skills in mathematical concepts and applications. This knowledge will be instrumental in navigating through more complex mathematical concepts and applications in the IB Mathematics curriculum. Further reading on this topic can be found in our introduction to sigma notation.

FAQ

Sigma notation is specifically designed for representing sums, not products. However, there is a similar notation for products, known as Pi notation, symbolised by the Greek letter Pi (Π). In Pi notation, the expression Π (from n=1 to k) f(n) represents the product of the terms f(n) as n varies from 1 to k. Just like sigma notation provides a concise way to represent sums, Pi notation provides a concise way to represent products, which can be particularly useful in mathematical proofs and calculus.

The comparison test is a method used to determine the convergence or divergence of a series by comparing it to another series whose convergence properties are known. If 0 ≤ an ≤ bn for all n in some range, and Sum (from n=1 to ∞) bn is convergent, then Sum (from n=1 to ∞) an is also convergent. Conversely, if Sum (from n=1 to ∞) bn is divergent, then Sum (from n=1 to ∞) an is also divergent. This test is particularly useful when dealing with series that are complex or difficult to evaluate directly, as it allows us to determine their convergence properties by comparing them to simpler series.

In probability theory, sigma notation is often used to represent the expected value (mean) of a discrete random variable. The expected value E(X) of a discrete random variable X is given by E(X) = Sum (from x=1 to n) [x * P(X=x)], where P(X=x) is the probability that X takes on the value x. Sigma notation is also used to represent other probability distributions and statistical measures, providing a concise way to represent sums of probabilities and enabling mathematicians and statisticians to work with probability distributions in a more manageable format.

In the context of sigma notation, a finite series is a sum that has a specific, finite number of terms. The upper limit of the sum is a specific integer. For example, Sum (from n=1 to 5) n2 is a finite series because it sums the squares of the first five natural numbers. On the other hand, an infinite series, such as Sum (from n=1 to ∞) 1/n2, has an infinite number of terms because it continues indefinitely (n goes to infinity). Understanding whether a series is finite or infinite is crucial as it impacts the methods used to evaluate the series and determine its convergence.

The change of variable technique in sigma notation involves substituting a new variable in place of the existing variable in the sum. This is often used to simplify the expression inside the sum or to align the limits of summation with the expression being summed. For example, if we have a sum S = Sum (from n=2 to 5) f(n), and we let m = n - 1, then our sum becomes S = Sum (from m=1 to 4) f(m + 1). This technique is particularly useful when dealing with nested sums or when trying to simplify the expression inside the sum to make it easier to evaluate.

Practice Questions

Evaluate the Sum

Evaluate the sum S = Sum (from n=1 to 4) n3.

The sum S = Sum (from n=1 to 4) n3 can be evaluated by substituting each value of n from 1 to 4 and adding the resulting terms. So, we substitute n = 1, 2, 3, and 4 into n3 and add them together:

S = 13 + 23 + 33 + 43 S = 1 + 8 + 27 + 64 S = 100

Thus, the sum S of n3 from n=1 to 4 is 100.

Divergent Series

Evaluate the sum S = Sum (from n=1 to ∞) 1/n.

The sum S = Sum (from n=1 to ∞) 1/n is known as the harmonic series. The harmonic series is a famous example of a divergent series, which means it does not have a finite sum. As n approaches infinity, the sum continues to grow without bound and does not approach a finite value. Therefore, the sum S = Sum (from n=1 to ∞) 1/n is infinite, or in mathematical terms, S diverges.

In these questions, students are asked to evaluate sums, which involves substituting values and adding terms together, and to recognise properties of well-known series. Understanding the properties of convergence and divergence, especially in the context of infinite series, is crucial for navigating through more complex mathematical concepts and applications in the IB Mathematics curriculum.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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