# 13.4 Series and their notations  (Page 3/18)

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A man earns $100 in the first week of June. Each week, he earns$12.50 more than the previous week. After 12 weeks, how much has he earned?

$2,025 ## Using the formula for geometric series Just as the sum of the terms of an arithmetic sequence is called an arithmetic series, the sum of the terms in a geometric sequence is called a geometric series . Recall that a geometric sequence is a sequence in which the ratio of any two consecutive terms is the common ratio , $\text{\hspace{0.17em}}r.\text{\hspace{0.17em}}$ We can write the sum of the first $n$ terms of a geometric series as ${S}_{n}={a}_{1}+r{a}_{1}+{r}^{2}{a}_{1}+...+{r}^{n–1}{a}_{1}.$ Just as with arithmetic series, we can do some algebraic manipulation to derive a formula for the sum of the first $\text{\hspace{0.17em}}n\text{\hspace{0.17em}}$ terms of a geometric series. We will begin by multiplying both sides of the equation by $\text{\hspace{0.17em}}r.\text{\hspace{0.17em}}$ $r{S}_{n}=r{a}_{1}+{r}^{2}{a}_{1}+{r}^{3}{a}_{1}+...+{r}^{n}{a}_{1}$ Next, we subtract this equation from the original equation. Notice that when we subtract, all but the first term of the top equation and the last term of the bottom equation cancel out. To obtain a formula for ${S}_{n},$ divide both sides by $\left(1-r\right).$ ## Formula for the sum of the first n Terms of a geometric series A geometric series is the sum of the terms in a geometric sequence. The formula for the sum of the first $\text{\hspace{0.17em}}n\text{\hspace{0.17em}}$ terms of a geometric sequence is represented as Given a geometric series, find the sum of the first n terms. 1. Identify $\text{\hspace{0.17em}}{a}_{1},\text{\hspace{0.17em}}r,\text{\hspace{0.17em}}\text{and}\text{\hspace{0.17em}}n.$ 2. Substitute values for $\text{\hspace{0.17em}}{a}_{1},\text{\hspace{0.17em}}r,$ and $n$ into the formula ${S}_{n}=\frac{{a}_{1}\left(1–{r}^{n}\right)}{1–r}.$ 3. Simplify to find ${S}_{n}.$ ## Finding the first n Terms of a geometric series Use the formula to find the indicated partial sum of each geometric series. 1. ${S}_{11}$ for the series 2. $\underset{6}{\overset{k=1}{{\sum }^{\text{​}}}}3\cdot {2}^{k}$ 1. ${a}_{1}=8,$ and we are given that $n=11.$ We can find $r$ by dividing the second term of the series by the first. $r=\frac{-4}{8}=-\frac{1}{2}$ Substitute values for into the formula and simplify. $\begin{array}{l}{S}_{n}=\frac{{a}_{1}\left(1-{r}^{n}\right)}{1-r}\hfill \\ {S}_{11}=\frac{8\left(1-{\left(-\frac{1}{2}\right)}^{11}\right)}{1-\left(-\frac{1}{2}\right)}\approx 5.336\hfill \end{array}$ 2. Find ${a}_{1}$ by substituting $k=1$ into the given explicit formula. ${a}_{1}=3\cdot {2}^{1}=6$ We can see from the given explicit formula that $r=2.$ The upper limit of summation is 6, so $n=6.$ Substitute values for ${a}_{1},\text{\hspace{0.17em}}r,$ and $n$ into the formula, and simplify. $\begin{array}{l}{S}_{n}=\frac{{a}_{1}\left(1-{r}^{n}\right)}{1-r}\hfill \\ {S}_{6}=\frac{6\left(1-{2}^{6}\right)}{1-2}=378\hfill \end{array}$ Use the formula to find the indicated partial sum of each geometric series. ${S}_{20}$ for the series $\approx 2,000.00$ $\sum _{k=1}^{8}{3}^{k}$ 9,840 ## Solving an application problem with a geometric series At a new job, an employee’s starting salary is$26,750. He receives a 1.6% annual raise. Find his total earnings at the end of 5 years.

The problem can be represented by a geometric series with ${a}_{1}=26,750\text{;}\text{\hspace{0.17em}}$ $n=5\text{;}\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}r=1.016.$ Substitute values for $\text{\hspace{0.17em}}{a}_{1}\text{,}\text{\hspace{0.17em}}$ $r\text{,}$ and $n$ into the formula and simplify to find the total amount earned at the end of 5 years.

$\begin{array}{l}{S}_{n}=\frac{{a}_{1}\left(1-{r}^{n}\right)}{1-r}\hfill \\ {S}_{5}=\frac{26\text{,}750\left(1-{1.016}^{5}\right)}{1-1.016}\approx 138\text{,}099.03\hfill \end{array}$

He will have earned a total of $138,099.03 by the end of 5 years. At a new job, an employee’s starting salary is$32,100. She receives a 2% annual raise. How much will she have earned by the end of 8 years?

\$275,513.31

## Using the formula for the sum of an infinite geometric series

Thus far, we have looked only at finite series. Sometimes, however, we are interested in the sum of the terms of an infinite sequence rather than the sum of only the first $n$ terms. An infinite series    is the sum of the terms of an infinite sequence. An example of an infinite series is $2+4+6+8+...$

answer and questions in exercise 11.2 sums
what is a algebra
what is the identity of 1-cos²5x equal to?
__john __05
Kishu
Hi
Abdel
hi
Ye
hi
Nokwanda
C'est comment
Abdel
Hi
Amanda
hello
SORIE
Hiiii
Chinni
hello
Ranjay
hi
ANSHU
hiiii
Chinni
h r u friends
Chinni
yes
Hassan
so is their any Genius in mathematics here let chat guys and get to know each other's
SORIE
I speak French
Abdel
okay no problem since we gather here and get to know each other
SORIE
hi im stupid at math and just wanna join here
Yaona
lol nahhh none of us here are stupid it's just that we have Fast, Medium, and slow learner bro but we all going to work things out together
SORIE
it's 12
what is the function of sine with respect of cosine , graphically
tangent bruh
Steve
cosx.cos2x.cos4x.cos8x
sinx sin2x is linearly dependent
what is a reciprocal
The reciprocal of a number is 1 divided by a number. eg the reciprocal of 10 is 1/10 which is 0.1
Shemmy
Reciprocal is a pair of numbers that, when multiplied together, equal to 1. Example; the reciprocal of 3 is ⅓, because 3 multiplied by ⅓ is equal to 1
Jeza
each term in a sequence below is five times the previous term what is the eighth term in the sequence
I don't understand how radicals works pls
How look for the general solution of a trig function
stock therom F=(x2+y2) i-2xy J jaha x=a y=o y=b
sinx sin2x is linearly dependent
cr
root under 3-root under 2 by 5 y square
The sum of the first n terms of a certain series is 2^n-1, Show that , this series is Geometric and Find the formula of the n^th
cosA\1+sinA=secA-tanA
Wrong question
why two x + seven is equal to nineteen.
The numbers cannot be combined with the x
Othman
2x + 7 =19
humberto
2x +7=19. 2x=19 - 7 2x=12 x=6
Yvonne
because x is 6
SAIDI