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  • 466 6 SEQUENCES, SERIES, AND PROBABILITY

    Section 6-3 Arithmetic and Geometric Sequences

    Arithmetic and Geometric Sequencesnth-Term FormulasSum Formulas for Finite Arithmetic SeriesSum Formulas for Finite Geometric SeriesSum Formula for Infinite Geometric Series

    For most sequences it is difficult to sum an arbitrary number of terms of thesequence without adding term by term. But particular types of sequences, arith-metic sequences and geometric sequences, have certain properties that lead to con-venient and useful formulas for the sums of the corresponding arithmetic seriesand geometric series.

    Arithmetic and Geometric Sequences

    The sequence 5, 7, 9, 11, 13, . . . , 5 1 2(n 2 1), . . . , where each term after thefirst is obtained by adding 2 to the preceding term, is an example of an arithmeticsequence. The sequence 5, 10, 20, 40, 80, . . . , 5 (2)n21, . . . , where each termafter the first is obtained by multiplying the preceding term by 2, is an exampleof a geometric sequence.

    ARITHMETIC SEQUENCE

    A sequence

    a1, a2, a3, . . . , an, . . .

    is called an arithmetic sequence, or arithmetic progression, if thereexists a constant d, called the common difference, such that

    an 2 an21 5 d

    That is,

    an 5 an21 1 d for every n . 1

    GEOMETRIC SEQUENCE

    A sequence

    a1, a2, a3, . . . , an , . . .

    is called a geometric sequence, or geometric progression, if there existsa nonzero constant r, called the common ratio, such that

    D E F I N I T I O N

    1

    D E F I N I T I O N

    2

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    1

    6-3 Arithmetic and Geometric Sequences 467

    That is,

    an 5 ran21 for every n . 1

    (A) Graph the arithmetic sequence 5, 7, 9, . . . .Describe the graphs of all arithmetic sequences with commondifference 2.

    (B) Graph the geometric sequence 5, 10, 20, . . . .Describe the graphs of all geometric sequences with common ratio 2.

    Recognizing Arithmetic and Geometric Sequences

    Which of the following can be the first four terms of an arithmetic sequence?Of a geometric sequence?

    (A) 1, 2, 3, 5, . . . (B) 21, 3, 29, 27, . . .

    (C) 3, 3, 3, 3, . . . (D) 10, 8.5, 7, 5.5, . . .

    S o l u t i o n s (A) Since 2 2 1 5 2 3, there is no common difference, so the sequence isnot an arithmetic sequence. Since , there is no common ratio, so thesequence is not geometric either.

    (B) The sequence is geometric with common ratio 23, but it is not arithmetic.

    (C) The sequence is arithmetic with common difference 0 and it is also geometricwith common ratio 1.

    (D) The sequence is arithmetic with common difference 21.5, but it is notgeometric.

    Which of the following can be the first four terms of an arithmetic sequence? Ofa geometric sequence?

    (A) 8, 2, 0.5, 0.125, . . . (B) 27, 22, 3, 8, . . . (C) 1, 5, 25, 100, . . .

    nth-Term Formulas

    If {an} is an arithmetic sequence with common difference d, then

    a2 5 a1 1 d

    a3 5 a2 1 d 5 a1 1 2d

    a4 5 a3 1 d 5 a1 1 3d

    M A T C H E D P R O B L E M

    1

    21

    32

    E X A M P L E

    1

    anan21

    5 rD E F I N I T I O N

    2continued

  • T H E O R E M

    2

    T H E O R E M

    1

    468 6 SEQUENCES, SERIES, AND PROBABILITY

    This suggests Theorem 1, which can be proved by mathematical induction (seeProblem 63 in Exercise 6-3).

    THE nTH TERM OF AN ARITHMETIC SEQUENCE

    an 5 a1 1 (n 2 1)d for every n . 1

    Similarly, if {an} is a geometric sequence with common ratio r, then

    a2 5 a1r

    a3 5 a2r 5 a1r2

    a4 5 a3r 5 a1r3

    This suggests Theorem 2, which can also be proved by mathematical induction(see Problem 69 in Exercise 6-3).

    THE nTH TERM OF A GEOMETRIC SEQUENCE

    an 5 a1rn21 for every n . 1

    Finding Terms in Arithmetic and Geometric Sequences

    (A) If the first and tenth terms of an arithmetic sequence are 3 and 30, respec-tively, find the fiftieth term of the sequence.

    (B) If the first and tenth terms of a geometric sequence are 1 and 4, find theseventeenth term to three decimal places.

    S o l u t i o n s (A) First use Theorem 1 with a1 5 3 and Now find a50:a10 5 30 to find d:

    a50 5 a1 1 (50 2 1)3

    5 3 1 49 ? 3

    5 150

    (B) First let n 5 10, a1 5 1, a10 5 4 and use Theorem 2 to find r.

    an 5 a1rn21

    4 5 1r1021

    r 5 41/9

    d 5 3

    30 5 3 1 9d

    a10 5 a1 1 (10 2 1)d

    an 5 a1 1 (n 2 1)d

    E X A M P L E

    2

  • T H E O R E M

    4

    T H E O R E M

    3

    6-3 Arithmetic and Geometric Sequences 469

    Now use Theorem 2 again, this time with n 5 17.

    a17 5 a1r17 5 1 (41/9)17 5 417/9 13.716

    (A) If the first and fifteenth terms of an arithmetic sequence are 25 and 23,respectively, find the seventy-third term of the sequence.

    (B) Find the eighth term of the geometric sequence

    Sum Formulas for Finite Arithmetic Series

    If a1, a2, a3, . . . , an is a finite arithmetic sequence, then the corresponding seriesa1 1 a2 1 a3 1 . . . 1 an is called an arithmetic series. We will derive two sim-ple and very useful formulas for the sum of an arithmetic series. Let d be thecommon difference of the arithmetic sequence a1, a2, a3, . . . , an and let Sn denotethe sum of the series a1 1 a2 1 a3 1 . . . 1 an.

    Then

    Sn 5 a1 1 (a1 1 d ) 1 . . . 1 [a1 1 (n 2 2)d ] 1 [a1 1 (n 2 1)d ]

    Reversing the order of the sum, we obtain

    Sn 5 [a1 1 (n 2 1)d ] 1 [a1 1 (n 2 2)d ] 1 . . . 1 (a1 1 d ) 1 a1

    Adding the left sides of these two equations and corresponding elements of theright sides, we see that

    2Sn 5 [2a1 1 (n 2 1)d ] 1 [2a1 1 (n 2 1)d ] 1 . . . 1 [2a1 1 (n 2 1)d ]

    5 n[2a1 1 (n 2 1)d ]

    This can be restated as in Theorem 3.

    SUM OF AN ARITHMETIC SERIESFIRST FORM

    Sn 5 [2a1 1 (n 2 1)d ]

    By replacing a1 1 (n 2 1)d with an, we obtain a second useful formula forthe sum.

    SUM OF AN ARITHMETIC SERIESSECOND FORM

    Sn 5 (a1 1 an)n

    2

    n

    2

    1

    64, 2

    1

    32,

    1

    16, . . . .

    M A T C H E D P R O B L E M

    2

  • 470 6 SEQUENCES, SERIES, AND PROBABILITY

    The proof of the first sum formula by mathematical induction is left as anexercise (see Problem 64 in Exercise 6-3).

    Finding the Sum of an Arithmetic Series

    Find the sum of the first 26 terms of an arithmetic series if the first term is27 and d 5 3.

    S o l u t i o n Let n 5 26, a1 5 27, d 5 3, and use Theorem 3.

    Sn 5 [2a1 1 (n 2 1)d ]

    S26 5 [2(27) 1 (26 2 1)3]

    5 793

    Find the sum of the first 52 terms of an arithmetic series if the first term is 23and d 5 22.

    Finding the Sum of an Arithmetic Series

    Find the sum of all the odd numbers between 51 and 99, inclusive.

    S o l u t i o n First, use a1 5 51, an 5 99, and Now use Theorem 4 to find S25:Theorem 1 to find n:

    an 5 a1 1 (n 2 1)d Sn 5 (a1 1 an)

    99 5 51 1 (n 2 1)2 S25 5 (51 1 99)

    n 5 25 5 1,875

    Find the sum of all the even numbers between 222 and 52, inclusive.

    Prize Money

    A 16-team bowling league has $8,000 to be awarded as prize money. If thelast-place team is awarded $275 in prize money and the award increases bythe same amount for each successive finishing place, how much will the first-place team receive?

    E X A M P L E

    5

    M A T C H E D P R O B L E M

    4

    252

    n

    2

    E X A M P L E

    4

    M A T C H E D P R O B L E M

    3

    262

    n

    2

    E X A M P L E

    3

  • T H E O R E M

    5

    6-3 Arithmetic and Geometric Sequences 471

    S o l u t i o n If a1 is the award for the first-place team, a2 is the award for the second-placeteam, and so on, then the prize money awards form an arithmetic sequence withn 5 16, a16 5 275, and S16 5 8,000. Use Theorem 4 to find a1.

    Sn 5 (a1 1 an)

    8,000 5 (a1 1 275)

    a1 5 725

    Thus, the first-place team receives $725.

    Refer to Example 5. How much prize money is awarded to the second-place team?

    Sum Formulas for Finite Geometric Series

    If a1, a2, a3, . . . , an is a finite geometric sequence, then the corresponding seriesa1 1 a2 1 a3 1 . . . 1 an is called a geometric series. As with arithmetic series,we can derive two simple and very useful formulas for the sum of a geometricseries. Let r be the common ratio of the geometric sequence a1, a2, a3, . . . , anand let Sn denote the sum of the series a1 1 a2 1 a3 1 . . . 1 an. Then

    Sn 5 a1 1 a1r 1 a1r2 1 a1r

    3 1 . . . 1 a1rn22 1 a1r

    n21

    Multiply both sides of this equation by r to obtain

    rSn 5 a1r 1 a1r2 1 a1r

    3 1 . . . 1 a1rn21 1 a1r

    n

    Now subtract the left side of the second equation from the left side of the first,and the right side of the second equation from the right side of the first to obtain

    Sn 2 rSn 5 a1 2 a1rn

    Sn(1 2 r) 5 a1 2 a1rn

    Thus, solving for Sn, we obtain the following formula for the sum of a geomet-ric series:

    SUM OF A GEOMETRIC SERIESFIRST FORM

    Sn 5 r 1a1 2 a1r

    n

    1 2 r

    M A T C H E D P R O B L E M

    5

    162

    n

    2

  • T H E O R E M

    6

    472 6 SEQUENCES, SERIES, AND PROBABILITY

    Since an 5 a1rn21, or ran 5 a1r

    n, the sum formula also can be written in thefollowing form:

    SUM OF A GEOMETRIC SERIESSECOND FORM

    Sn 5 r 1

    The proof of the first sum formula (Theorem 5) by mathematical induction isleft as an exercise (see Problem 70, Exercise 6-3).

    If r 5 1, then

    Sn 5 a1 1 a1(1) 1 a1(12) 1 . . . 1 a1(1

    n21) 5 na1

    Finding the Sum of a Geometric Series

    Find the sum of the first 20 terms of a geometric series if the first term is 1and r 5 2.

    S o l u t i o n Let n 5 20, a1 5 1, r 5 2, and use Theorem 5.

    Calculation using a calculator

    Find the sum, to two decimal places, of the first 14 terms of a geometric seriesif the first term is and r 5 22.

    Sum Formula for Infinite Geometric Series

    Consider a geometric series with a1 5 5 and r 5 . What happens to the sum Snas n increases? To answer this question, we first write the sum formula in themore convenient form

    (1)

    For a1 5 5 and r 5 ,

    Sn 5 10 2 101122n

    12

    Sn 5a1 2 a1r

    n

    1 2 r5

    a11 2 r

    2a1r

    n

    1 2 r

    12

    164

    M A T C H E D P R O B L E M

    6

    51 2 1 ? 220

    1 2 25 1,048,575

    Sn 5a1 2 a1r

    n

    1 2 r

    E X A M P L E

    6

    a1 2 ran1 2 r

  • 6-3 Arithmetic and Geometric Sequences 473

    Thus,

    It appears that becomes smaller and smaller as n increases and that the sumgets closer and closer to 10.

    In general, it is possible to show that, if , 1, then rn will get closer andcloser to 0 as n increases. Symbolically, rn 0 as n `. Thus, the term

    in equation (1) will tend to 0 as n increases, and Sn will tend to

    In other words, if , 1, then Sn can be made as close to

    as we wish by taking n sufficiently large. Thus, we define the sum of an infinitegeometric series by the following formula:

    SUM OF AN INFINITE GEOMETRIC SERIES

    S` 5

    If $ 1, an infinite geometric series has no sum.

    Expressing a Repeating Decimal as a Fraction

    Represent the repeating decimal 0.454 545 . . . 5 as the quotient of twointegers. Recall that a repeating decimal names a rational number and that anyrational number can be represented as the quotient of two integers.

    0.45

    E X A M P L E

    7

    |r|

    a11 2 r

    |r| , 1

    a11 2 r

    |r|

    a11 2 r

    a1rn

    1 2 r

    |r|(12)

    n

    S20 5 10 2 101 11,048,5762

    S10 5 10 2 101 11,0242

    S4 5 10 2 101 1162

    S2 5 10 2 101142

    D E F I N I T I O N

    3

  • Explore/Discuss

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    474 6 SEQUENCES, SERIES, AND PROBABILITY

    S o l u t i o n 5 0.45 1 0.0045 1 0.000 045 1 . . .

    The right side of the equation is an infinite geometric series with a1 5 0.45 andr 5 0.01. Thus,

    S` 5

    Hence, and name the same rational number. Check the result by dividing5 by 11.

    Repeat Example 7 for 0.818 181 . . . 5 .

    Economy Stimulation

    A state government uses proceeds from a lottery to provide a tax rebate forproperty owners. Suppose an individual receives a $500 rebate and spends80% of this, and each of the recipients of the money spent by this individualalso spends 80% of what he or she receives, and this process continues with-out end. According to the multiplier doctrine in economics, the effect of theoriginal $500 tax rebate on the economy is multiplied many times. What isthe total amount spent if the process continues as indicated?

    S o l u t i o n The individual receives $500 and spends 0.8(500) 5 $400. The recipients of this$400 spend 0.8(400) 5 $320, the recipients of this $320 spend 0.8(320) 5 $256,and so on. Thus, the total spending generated by the $500 rebate is

    400 1 320 1 256 1 . . . 5 400 1 0.8(400) 1 (0.8)2(400) 1 . . .

    which we recognize as an infinite geometric series with a1 5 400 and r 5 0.8.Thus, the total amount spent is

    S` 5

    Repeat Example 8 if the tax rebate is $1,000 and the percentage spent by all recip-ients is 90%.

    (A) Find an infinite geometric series with a1 5 10 whose sum is 1,000.

    (B) Find an infinite geometric series with a1 5 10 whose sum is 6.

    (C) Suppose that an infinite geometric series with a1 5 10 has a sum.Explain why that sum must be greater than 5.

    M A T C H E D P R O B L E M

    8

    a11 2 r

    5400

    1 2 0.85

    400

    0.25 $2,000

    E X A M P L E

    8

    0.81M A T C H E D P R O B L E M

    7

    5110.45

    a11 2 r

    50.45

    1 2 0.015

    0.45

    0.995

    5

    11

    0.45

  • 6-3 Arithmetic and Geometric Sequences 475

    A n s w e r s t o M a t c h e d P r o b l e m s

    1. (A) The sequence is geometric with r 5 , but not arithmetic.(B) The sequence is arithmetic with d 5 5, but not geometric.(C) The sequence is neither arithmetic nor geometric.

    2. (A) 139 (B) 22 3. 21,456 4. 570 5. $695 6. 285.33 7. 8. $9,000911

    14

    EXERCISE 6-3

    A

    In Problems 1 and 2, determine whether the following can bethe first three terms of an arithmetic or geometric sequence,and, if so, find the common difference or common ratio and thenext two terms of the sequence.

    1. (A) 211, 216, 221, . . . (B) 2, 24, 8, . . .(C) 1, 4, 9, . . . (D) , . . .

    2. (A) 5, 20, 100, . . . (B) 25, 25, 25, . . .(C) 7, 6.5, 6, . . . (D) 512, 256, 128, . . .

    Let a1, a2, a3, . . . , an, . . . be an arithmetic sequence. InProblems 310, find the indicated qualities.

    3. a1 5 25, d 5 4; a2 5 ?, a3 5 ?, a4 5 ?

    4. a1 5 218, d 5 3; a2 5 ?, a3 5 ?, a4 5 ?

    5. a1 5 23, d 5 5; a15 5 ?, S11 5 ?

    6. a1 5 3, d 5 4; a22 5 ?, S21 5 ?

    7. a1 5 1, a2 5 5; S21 5 ?

    8. a1 5 5, a2 5 11; S11 5 ?

    9. a1 5 7, a2 5 5; a15 5 ?

    10. a1 5 23, d 5 24; a10 5 ?

    Let a1, a2, a3, . . . , an, . . . be a geometric sequence. InProblems 1116, find each of the indicated quantities.

    11. a1 5 26, r 5 2 ; a2 5 ?, a3 5 ?, a4 5 ?

    12. a1 5 12, r 5 ; a2 5 ?, a3 5 ?, a4 5 ?

    13. a1 5 81, r 5 ; a10 5 ?

    14. a1 5 64, r 5 ; a13 5 ?

    15. a1 5 3, a7 5 2,187, r 5 3; S7 5 ?

    16. a1 5 1, a7 5 729, r 5 23; S7 5 ?

    B

    Let a1, a2, a3, . . . , an, . . . be an arithmetic sequence. InProblems 1724, find the indicated quantities.

    12

    13

    23

    12

    12,

    16,

    118

    17. a1 5 3, a20 5 117; d 5 ?, a101 5 ?

    18. a1 5 7, a8 5 28; d 5 ?, a25 5 ?

    19. a1 5 212, a40 5 22; S40 5 ?

    20. a1 5 24, a24 5 228; S24 5 ?

    21. a1 5 , a2 5 ; a11 5 ?, S11 5 ?

    22. a1 5 , a2 5 ; a19 5 ?, S19 5 ?

    23. a3 5 13, a10 5 55; a1 5 ?

    24. a9 5 212, a13 5 3; a1 5 ?

    Let a1, a2, a3, . . . , an, . . . be a geometric sequence. Find eachof the indicated quantities in Problems 2530.

    25. a1 5 100, a6 5 1; r 5 ? 26. a1 5 10, a10 5 30; r 5 ?

    27. a1 5 5, r 5 22; S10 5 ? 28. a1 5 3, r 5 2; S10 5 ?

    29. a1 5 9, a4 5 ; a2 5 ?, a3 5 ?

    30. a1 5 12, a4 5 2 ; a2 5 ?, a3 5 ?

    31. S51 5 5 ? 32. S40 5 5 ?

    33. S7 5 5 ? 34. S7 5 5 ?

    35. Find g(1) 1 g(2) 1 g(3) 1 . . . 1 g(51) if g(t) 5 5 2 t.

    36. Find f(1) 1 f(2) 1 f(3) 1 . . . 1 f(20) if f(x) 5 2x 2 5.

    37. Find g(1) 1 g(2) 1 . . . 1 g(10) if g(x) 5 .

    38. Find f(1) 1 f(2) 1 . . . 1 f(10) if f(x) 5 2x.

    39. Find the sum of all the even integers between 21 and 135.

    40. Find the sum of all the odd integers between 100 and 500.

    41. Show that the sum of the first n odd natural numbers is n2,using approximate formulas from this section.

    42. Show that the sum of the first n even natural numbers is n 1 n2, using appropriate formulas from this section.

    43. Find a positive number x so that 22 1 x 2 6 is a three-term geometric series.

    44. Find a positive number x so that 6 1 x 1 8 is a three-termgeometric series.

    45. For a given sequence in which a1 5 23 and an 5 an21 1 3,n . 1, find an in terms of n.

    (12)x

    o7

    k51

    3ko7

    k51

    (23)k21

    o40

    k51

    (2k 2 3)o51

    k51

    (3k 1 3)

    49

    83

    14

    16

    12

    13

  • 476 6 SEQUENCES, SERIES, AND PROBABILITY

    46. For the sequence in Problem 45, find Sn 5 ak in terms of n.

    In Problems 4750, find the least positive integer n such thatan , bn by graphing the sequences {an} and {bn} with agraphing utility. Check your answer by using a graphing utilityto display both sequences in table form.

    47. an 5 5 1 8n, bn 5 1.1n

    48. an 5 96 1 47n, bn 5 8(1.5)n

    49. an 5 1,000 (0.99)n, bn 5 2n 1 1

    50. an 5 500 2 n, bn 5 1.05n

    In Problems 5156, find the sum of each infinite geometricseries that has a sum.

    51. 3 1 1 1 1 . . . 52. 16 1 4 1 1 1 . . .

    53. 2 1 4 1 8 1 . . . 54. 4 1 6 1 9 1 . . .

    55. 2 2 1 2 . . . 56. 21 2 3 1 2 . . .

    In Problems 5762, represent each repeating decimal as thequotient of two integers.

    57. 5 0.7777 . . . 58. 5 0.5555 . . .

    59. 5 0.545 454 . . . 60. 5 0.272 727 . . .

    61. 5 3.216 216 216 . . .

    62. 5 5.636 363 . . .

    C

    63. Prove, using mathemtical induction, that if {an} is anarithmetic sequence, then

    an 5 a1 1 (n 2 1)d for every n . 1

    64. Prove, using mathematical induction, that if {an} is anarithmetic sequence, then

    Sn 5 [2a1 1 (n 2 1)d]

    65. If in a given sequence, a1 5 22 and an 5 23an21, n . 1,find an in terms of n.

    66. For the sequence in Problem 65, find Sn 5 ak in terms of n.

    67. Show that (x2 1 xy 1 y2), (z2 1 xz 1 x2), and (y2 1 yz 1z2) are consecutive terms of an arithmetic progression if x,y, and z form an arithmetic progression. (From U.S.S.R.Mathematical Olympiads, 19551956, Grade 9.)

    68. Take 121 terms of each arithmetic progression 2, 7,12, . . . and 2, 5, 8, . . . . How many numbers will there bein common? (From U.S.S.R. Mathematical Olympiads,19551956, Grade 9.)

    on

    k51

    n

    2

    5.63

    3.216

    0.270.54

    0.50.7

    37

    18

    12

    13

    on

    k5169. Prove, using mathematical induction, that if {an} is a geo-

    metric sequence, then

    an 5 a1rn21 n [ N

    70. Prove, using mathematical induction, that if {an} is a geo-metric sequence, then

    Sn 5 n [ N, r 1

    71. Given the system of equations

    ax 1 by 5 c

    dx 1 ey 5 f

    where a, b, c, d, e, and f is any arithmetic progression witha nonzero constant difference, show that the system has aunique solution.

    72. The sum of the first and fourth terms of an arithmetic se-quence is 2, and the sum of their squares is 20. Find thesum of the first eight terms of the sequence.

    APPLICATIONS

    73. Business. In investigating different job opportunities, youfind that firm A will start you at $25,000 per year andguarantee you a raise of $1,200 each year while firm Bwill start you at $28,000 per year but will guarantee you araise of only $800 each year. Over a period of 15 years,how much would you receive from each firm?

    74. Business. In Problem 73, what would be your annualsalary at each firm for the tenth year?

    75. Economics. The government, through a subsidy program,distributes $1,000,000. If we assume that each individualor agency spends 0.8 of what is received, and 0.8 of this isspent, and so on, how much total increase in spending re-sults from this government action?

    76. Economics. Due to reduced taxes, an individual has anextra $600 in spendable income. If we assume that the in-dividual spends 70% of this on consumer goods, that theproducers of these goods in turn spend 70% of what theyreceive on consumer goods, and that this process contin-ues indefinitely, what is the total amount spent on con-sumer goods?

    77. Business. If $P is invested at r% compounded annually,the amount A present after n years forms a geometric pro-gression with a common ratio 1 1 r. Write a formula forthe amount present after n years. How long will it take asum of money P to double if invested at 6% interest com-pounded annually?

    78. Population Growth. If a population of A0 people grows atthe constant rate of r% per year, the population after tyears forms a geometric progression with a common ratio1 1 r. Write a formula for the total population after tyears. If the worlds population is increasing at the rate of2% per year, how long will it take to double?

    a1 2 a1rn

    1 2 r

  • 6-3 Arithmetic and Geometric Sequences 477

    79. Finance. Eleven years ago an investment earned $7,000for the year. Last year the investment earned $14,000. Ifthe earnings from the investment have increased the sameamount each year, what is the yearly increase and howmuch income has accrued from the investment over thepast 11 years?

    80. Air Temperature. As dry air moves upward, it expands.In so doing, it cools at the rate of about 5F for each1,000-foot rise. This is known as the adiabatic process.

    (A) Temperatures at altitudes that are multiples of 1,000feet form what kind of a sequence?

    (B) If the ground temperature is 80F, write a formula forthe temperature Tn in terms of n, if n is in thousandsof feet.

    81. Engineering. A rotating flywheel coming to rest rotates300 revolutions the first minute (see the figure). If in eachsubsequent minute it rotates two-thirds as many times asin the preceding minute, how many revolutions will thewheel make before coming to rest?

    82. Physics. The first swing of a bob on a pendulum is 10inches. If on each subsequent swing it travels 0.9 as far ason the preceding swing, how far will the bob travel beforecoming to rest?

    83. Food Chain. A plant is eaten by an insect, an insect by atrout, a trout by a salmon, a salmon by a bear, and the bearis eaten by you. If only 20% of the energy is transformedfrom one stage to the next, how many calories must besupplied by plant food to provide you with 2,000 caloriesfrom the bear meat?

    84. Genealogy. If there are 30 years in a generation, howmany direct ancestors did each of us have 600 years ago?By direct ancestors we mean parents, grandparents, great-grandparents, and so on.

    85. Physics. An object falling from rest in a vacuum near thesurface of the Earth falls 16 feet during the first second, 48feet during the second second, 80 feet during the third sec-ond, and so on.

    (A) How far will the object fall during the eleventhsecond?

    (B) How far will the object fall in 11 seconds?

    (C) How far will the object fall in t seconds?

    86. Physics. In Problem 85, how far will the object fallduring:

    (A) The twentieth second? (B) The t th second?

    87. Bacteria Growth. A single cholera bacterium dividesevery hour to produce two complete cholera bacteria. Ifwe start with a colony of A0 bacteria, how many bacteriawill we have in t hours, assuming adequate food supply?

    88. Cell Division. One leukemic cell injected into a healthymouse will divide into two cells in about day. At the endof the day these two cells will divide again, with the dou-bling process continuing each day until there are 1 bil-lion cells, at which time the mouse dies. On which dayafter the experiment is started does this happen?

    89. Astronomy. Ever since the time of the Greek astronomerHipparchus, second century B.C., the brightness of starshas been measured in terms of magnitude. The brighteststars, excluding the sun, are classed as magnitude 1, andthe dimmest visible to the eye are classed as magnitude 6.In 1856, the English astronomer N. R. Pogson showed thatfirst-magnitude stars are 100 times brighter than sixth-magnitude stars. If the ratio of brightness between consec-utive magnitudes is constant, find this ratio. [Hint: If bn isthe brightness of an nth-magnitude star, find r for the geo-metric progression b1, b2, b3, . . . , given b1 5 100b6.]

    90. Music. The notes on a piano, as measured in cycles persecond, form a geometric progression.

    (A) If A is 400 cycles per second and A9, 12 notes higher,is 800 cycles per second, find the constant ratio r.

    12

    12

    12

  • 478 6 SEQUENCES, SERIES, AND PROBABILITY

    (B) Find the cycles per second for C, three notes higherthan A.

    91. Puzzle. If you place 1 on the first square of a chessboard,2 on the second square, 4 on the third, and so on, con-tinuing to double the amount until all 64 squares are cov-ered, how much money will be on the sixty-fourth square?How much money will there be on the whole board?

    92. Puzzle. If a sheet of very thin paper 0.001 inch thick istorn in half, and each half is again torn in half, and thisprocess is repeated for a total of 32 times, how high willthe stack of paper be if the pieces are placed one on top ofthe other? Give the answer to the nearest mile.

    93. Atmospheric Pressure. If atmospheric pressure decreasesroughly by a factor of 10 for each 10-mile increase in alti-

    tude up to 60 miles, and if the pressure is 15 pounds persquare inch at sea level, what will the pressure be 40 milesup?

    94. Zenos Paradox. Visualize a hypothetical 440-yard ovalracetrack that has tapes stretched across the track at thehalfway point and at each point that marks the halfwaypoint of each remaining distance thereafter. A runner run-ning around the track has to break the first tape before thesecond, the second before the third, and so on. From thispoint of view it appears that he will never finish the race.This famous paradox is attributed to the Greek philoso-pher Zeno (495435 B.C.). If we assume the runner runs at440 yards per minute, the times between tape breakingsform an infinite geometric progression. What is the sum ofthis progression?

    95. Geometry. If the midpoints of the sides of an equilateraltriangle are joined by straight lines, the new figure will bean equilateral triangle with a perimeter equal to half theoriginal. If we start with an equilateral triangle withperimeter 1 and form a sequence of nested equilateraltriangles proceeding as described, what will be the totalperimeter of all the triangles that can be formed in thisway?

    96. Photography. The shutter speeds and f-stops on a cameraare given as follows:

    Shutter speeds:

    f-stops: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22

    These are very close to being geometric progressions. Es-timate their common ratios.

    97. Geometry. We know that the sum of the interior angles ofa triangle is 180. Show that the sums of the interior an-gles of polygons with 3, 4, 5, 6, . . . sides form an arith-metic sequence. Find the sum of the interior angles for a21-sided polygon.

    1, 12, 14,

    18,

    115,

    130,

    160,

    1125,

    1250,

    1500

    Section 6-4 Multiplication Principle, Permutations,and Combinations

    Multiplication PrincipleFactorialPermutationsCombinations

    This section introduces some new mathematical tools that are usually referred toas counting techniques. In general, a counting technique is a mathematicalmethod of determining the number of objects in a set without actually enumerat-ing the objects in the set as 1, 2, 3, . . . . For example, we can count the number