In the previous section, we introduced sequences and now we shall present notation and theorems concerning the sum of terms of a sequence. We begin with a definition, which, while intimidating, is meant to make our lives easier.
a_ \nonumber\] The variable \(n\) is called the index of summation. The number \(m\) is called the lower limit of summation while the number \(p\) is called the upper limit of summation.
\). The symbol \(\Sigma\) is the capital Greek letter sigma and is shorthand for ‘sum’. The lower and upper limits of the summation tells us which term to start with and which term to end with, respectively. For example, using the sequence \(a_ = 2n-1\) for \(n \geq 1\), we can write the sum \(a_ +a_ + a_ + a_\) as \[\begin \displaystyle^(2n-1) > & = & (2(3)-1) + (2(4)-1) + (2(5)-1) + (2(6)-1) \\ & = & 5 + 7 + 9 + 11 \\ & = & 32 \\ \end\nonumber\] The index variable is considered a ‘dummy variable’ in the sense that it may be changed to any letter without affecting the value of the summation. For instance, \[\displaystyle^(2n-1)> = \displaystyle^(2k-1)> = \displaystyle^(2j-1)>\nonumber\] One place you may encounter summation notation is in mathematical definitions. For example, summation notation allows us to define polynomials as functions of the form \[f(x) = \displaystyle^ a_ x^>\nonumber\] for real numbers \(a_\), \(k = 0, 1, \ldots n\). The reader is invited to compare this with what is given in Definition 3.1. Summation notation is particularly useful when talking about matrix operations. For example, we can write the product of the \(i\)th row \(R_\) of a matrix \(A = [a_]_\) and the \(j^>\) column \(C_\) of a matrix \(B = [b_]_\) as \[Ri \cdot Cj = \displaystyle^ a_b_>\nonumber\] Again, the reader is encouraged to write out the sum and compare it to Definition 8.9. Our next example gives us practice with this new notation.
Solution
The following theorem presents some general properties of summation notation. While we shall not have much need of these properties in Algebra, they do play a great role in Calculus. Moreover, there is much to be learned by thinking about why the properties hold. We invite the reader to prove these results. To get started, remember, “When in doubt, write it out!”
Suppose \(\left\\right\>\) and \(\left\\right\>\) are sequences so that the following sums are defined.
\left(a_ \pm b_ \right) = \sum_^
a_ \pm \sum_^
b_ >\)
c \, a_ = c \sum_^
a_>\), for any real number \(c\).
a_ = \sum_^ a_ + \sum_^
a_>\), for any natural number \(m \leq j < j+1 \leq p\).
a_ = \sum_^ a_>\), for any whole number \(r\).
We now turn our attention to the sums involving arithmetic and geometric sequences. Given an arithmetic sequence \(a_ = a + (k-1) d\) for \(k \geq 1\), we let \(S\) denote the sum of the first \(n\) terms. To derive a formula for \(S\), we write it out in two different ways \[\begin S & = & a & + & (a + d) & + & \ldots & + & (a + (n-2)d) & + & (a + (n-1)d) \\ S & = & (a + (n-1)d) & + & (a + (n-2)d) & + & \ldots & + & (a + d) & + & a \\ \end\nonumber\] If we add these two equations and combine the terms which are aligned vertically, we get
\[2S = (2a + (n-1)d) + (2a + (n-1)d) + \ldots + (2a + (n-1)d) + (2a + (n-1)d)\nonumber\]
The right hand side of this equation contains \(n\) terms, all of which are equal to \((2a + (n-1)d)\) so we get \(2S = n(2a + (n-1)d)\). Dividing both sides of this equation by \(2\), we obtain the formula
\[S = \dfrac (2a + (n-1)d)\nonumber\]
If we rewrite the quantity \(2a + (n-1)d\) as \(a + (a + (n-1)d) = a_ + a_\), we get the formula
A helpful way to remember this last formula is to recognize that we have expressed the sum as the product of the number of terms \(n\) and the average of the first and \(n^>\) terms.
To derive the formula for the geometric sum, we start with a geometric sequence \(a_ = ar^\), \(k \geq 1\), and let \(S\) once again denote the sum of the first \(n\) terms. Comparing \(S\) and \(rS\), we get
\[\begin S & = & a & + & ar & + & ar^2 & + & \ldots & + & ar^ & + & ar^ & & \\ r S & = & & & ar & + & ar^2 & + & \ldots & + & ar^ & + & ar^ & + & ar^ \\ \end\nonumber\]
Subtracting the second equation from the first forces all of the terms except \(a\) and \(ar^\) to cancel out and we get \(S - rS = a - ar^\). Factoring, we get \(S(1-r) = a \left(1-r^\right)\). Assuming \(r \neq 1\), we can divide both sides by the quantity \((1-r)\) to obtain
\[S = a \left( \dfrac\right)\nonumber\]
If we distribute \(a\) through the numerator, we get \(a - ar^ = a_ - a_\) which yields the formula
In the case when \(r=1\), we get the formula
Our results are summarized below.
While we have made an honest effort to derive the formulas in Equation 9.2, formal proofs require the machinery in Section 9.3. An application of the arithmetic sum formula which proves useful in Calculus results in formula for the sum of the first \(n\) natural numbers. The natural numbers themselves are a sequence 4 \(1\), \(2\), \(3\), …which is arithmetic with \(a = d = 1\). Applying Equation 9.2,
\[\begin 1 + 2 + 3 + \ldots + n & = & \dfrac \end\nonumber\]
So, for example, the sum of the first \(100\) natural numbers 5 is \(\frac = 5050\).
An important application of the geometric sum formula is the investment plan called an annuity. Annuities differ from the kind of investments we studied in Section 6.5 in that payments are deposited into the account on an on-going basis, and this complicates the mathematics a little. 6 Suppose you have an account with annual interest rate \(r\) which is compounded \(n\) times per year. We let \(i = \frac\) denote the interest rate per period. Suppose we wish to make ongoing deposits of \(P\) dollars at the end of each compounding period. Let \(A_\) denote the amount in the account after \(k\) compounding periods. Then \(A_ = P\), because we have made our first deposit at the end of the first compounding period and no interest has been earned. During the second compounding period, we earn interest on \(A_\) so that our initial investment has grown to \(A_(1+i) = P(1+i)\) in accordance with Equation 6.1. When we add our second payment at the end of the second period, we get
\[A_2 = A_1(1+i) + P = P(1+i) + P = P(1+i)\left(1 + \dfrac\right)\nonumber\]
The reason for factoring out the \(P(1+i)\) will become apparent in short order. During the third compounding period, we earn interest on \(A_\) which then grows to \(A_(1+i)\). We add our third payment at the end of the third compounding period to obtain
\[A_3 = A_2(1+i) + P = P(1+i)\left(1 + \dfrac\right)(1+i) + P = P(1+i)^2\left(1 + \dfrac + \dfrac\right)\nonumber\]
During the fourth compounding period, \(A_\) grows to \(A_(1+i)\), and when we add the fourth payment, we factor out \(P(1+i)^3\) to get
This pattern continues so that at the end of the \(k\)th compounding, we get
The sum in the parentheses above is the sum of the first \(k\) terms of a geometric sequence with \(a = 1\) and \(r = \frac\). Using Equation 9.2, we get
If we let \(t\) be the number of years this investment strategy is followed, then \(k = nt\), and we get the formula for the future value of an ordinary annuity.
Suppose an annuity offers an annual interest rate \(r\) compounded \(n\) times per year. Let \(i = \frac\) be the interest rate per compounding period. If a deposit \(P\) is made at the end of each compounding period, the amount \(A\) in the account after \(t\) years is given by
The reader is encouraged to substitute \(i = \frac\) into Equation 9.3 and simplify. Some familiar equations arise which are cause for pause and meditation. One last note: if the deposit \(P\) is made a the beginning of the compounding period instead of at the end, the annuity is called an annuity-due. We leave the derivation of the formula for the future value of an annuity-due as an exercise for the reader.
An ordinary annuity offers a \(6 \%\) annual interest rate, compounded monthly.
Solution.
We close this section with a peek into Calculus by considering infinite sums, called series. Consider the number \(0.\overline\). We can write this number as
\[0.\overline = 0.9999. = 0.9 + 0.09 + 0.009 + 0.0009 + \ldots\nonumber\]
From Example 9.2.1, we know we can write the sum of the first \(n\) of these terms as
It stands to reason that \(0.\overline\) is the same value of \(1 - \frac>\) as \(n \rightarrow \infty\). Our knowledge of exponential expressions from Section 6.1 tells us that \(\frac> \rightarrow 0\) as \(n \rightarrow \infty\), so \(1 - \frac> \rightarrow 1\). We have just argued that \(0.\overline = 1\), which may cause some distress for some readers. 7 Any non-terminating decimal can be thought of as an infinite sum whose denominators are the powers of \(10\), so the phenomenon of adding up infinitely many terms and arriving at a finite number is not as foreign of a concept as it may appear. We end this section with a theorem concerning geometric series.
Given the sequence \(a_ = ar^\) for \(k \geq 1\), where \(|r| < 1\),
If \(|r| \geq 1\), the sum \(a + ar + ar^2 + \ldots\) is not defined.
The justification of the result in Theorem 9.2 comes from taking the formula in Equation 9.2 for the sum of the first \(n\) terms of a geometric sequence and examining the formula as \(n \rightarrow \infty\). Assuming \(|r| \rightarrow 0\) as \(n \rightarrow \infty\). Hence as \(n \rightarrow \infty\),
As to what goes wrong when \(|r| \geq 1\), we leave that to Calculus as well, but will explore some cases in the exercises.
In Exercises 1 - 8, find the value of each sum using Definition 9.3.
In Exercises 9 - 16, rewrite the sum using summation notation.
In Exercises 17 - 28, use the formulas in Equation 9.2 to find the sum.
In Exercises 29 - 32, use Theorem 9.2 to express each repeating decimal as a fraction of integers.
In Exercises 33 - 38, use Equation 9.3 to compute the future value of the annuity with the given terms. In all cases, assume the payment is made monthly, the interest rate given is the annual rate, and interest is compounded monthly.
1 This is indeed a geometric sequence with first term \(a = 1\) and common ratio \(r = −1\).
2 It is an arithmetic sequence with first term \(a = 1\) and common difference \(d = 1\).
3 To see why, try writing the summation using ‘\(n\)’ as the index.
4 This is the identity function on the natural numbers!
5 There is an interesting anecdote which says that the famous mathematician Carl Friedrich Gauss was given this problem in primary school and devised a very clever solution.
6 The reader may wish to re-read the discussion on compound interest in Section 6.5 before proceeding.
7 To make this more palatable, it is usually accepted that \(0 . \overline=\frac\) so that \(0 . \overline=3(0 . \overline)=3\left(\frac\right)=1\). 1. Feel better?
8 When in doubt, write them out!
This page titled 9.2: Summation Notation is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Carl Stitz & Jeff Zeager via source content that was edited to the style and standards of the LibreTexts platform.
