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QUESTION 2: Arithmetic and Geometric Sequences and Series. QUESTION 3: Quadratic Number Patterns and Series.

Mathematics

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Here are the solutions to the questions about series and sequences.

Question 2.1: Arithmetic Series

Given the arithmetic series: 18+24+30+⋯+30018 + 24 + 30 + \dots + 300.
The first term is a=18a = 18.
The common difference is d=24−18=6d = 24 - 18 = 6.
The last term is Tn=300T_n = 300.

2.1.1 Determine the number of terms in this series.

  1. Use the formula for the nthn^{th} term of an arithmetic series, Tn=a+(n−1)dT_n = a + (n-1)d.

    300=18+(n−1)6300 = 18 + (n-1)6
  2. Solve for nn.

    300−18=(n−1)6300 - 18 = (n-1)6 282=6(n−1)282 = 6(n-1) 2826=n−1\frac{282}{6} = n-1 47=n−147 = n-1 n=47+1n = 47 + 1 n=48n = 48

    The number of terms in the series is 48\boxed{48}.

    2.1.2 Calculate the sum of this series.

  3. Use the formula for the sum of an arithmetic series, Sn=n2(a+Tn)S_n = \frac{n}{2}(a + T_n).

    S48=482(18+300)S_{48} = \frac{48}{2}(18 + 300)
  4. Calculate the sum.

    S48=24(318)S_{48} = 24(318) S48=7632S_{48} = 7632

    The sum of this series is 7632\boxed{7632}.

    2.1.3 Calculate the sum of all the whole numbers up to and including 300 that are NOT divisible by 6.

  5. Calculate the sum of all whole numbers from 1 to 300.

    The sum of the first NN whole numbers is SN=N(N+1)2S_N = \frac{N(N+1)}{2}. Here N=300N=300.

    S300=300(300+1)2S_{300} = \frac{300(300+1)}{2} S300=300×3012S_{300} = \frac{300 \times 301}{2} S300=150×301S_{300} = 150 \times 301 S300=45150S_{300} = 45150
  6. Calculate the sum of all whole numbers from 1 to 300 that ARE divisible by 6.

    These numbers form an arithmetic series: 6,12,18,…,3006, 12, 18, \dots, 300.
    First term a′=6a' = 6. Common difference d′=6d' = 6. Last term Tk′=300T_k' = 300.
    Find the number of terms kk:

    Tk′=a′+(k−1)d′T_k' = a' + (k-1)d' 300=6+(k−1)6300 = 6 + (k-1)6 294=6(k−1)294 = 6(k-1) 2946=k−1\frac{294}{6} = k-1 49=k−149 = k-1 k=50k = 50

    Now, calculate the sum Sk′S_k':

    Sk′=k2(a′+Tk′)S_k' = \frac{k}{2}(a' + T_k') S50′=502(6+300)S_{50}' = \frac{50}{2}(6 + 300) S50′=25(306)S_{50}' = 25(306) S50′=7650S_{50}' = 7650
  7. Subtract the sum of numbers divisible by 6 from the sum of all numbers up to 300.

    Required Sum = (Sum of all numbers up to 300) - (Sum of numbers divisible by 6 up to 300)

    Required Sum=45150−7650\text{Required Sum} = 45150 - 7650 Required Sum=37500\text{Required Sum} = 37500

    The sum of all whole numbers up to and including 300 that are NOT divisible by 6 is 37500\boxed{37500}.

    Question 2.2: Geometric Sequence

    The first three terms of an infinite geometric sequence are 16, 8, and 4.
    The first term is a=16a = 16.
    The common ratio is r=816=12r = \frac{8}{16} = \frac{1}{2}.

    2.2.1 Determine the nthn^{th} term of the sequence.

  8. Use the formula for the nthn^{th} term of a geometric sequence, Tn=arn−1T_n = ar^{n-1}.

    Tn=16(12)n−1T_n = 16 \left(\frac{1}{2}\right)^{n-1}

    The nthn^{th} term of the sequence is Tn=16(12)n−1\boxed{T_n = 16 \left(\frac{1}{2}\right)^{n-1}}.

    2.2.2 Determine all possible values of nn for which the sum of the first nn terms of this sequence is greater than 31.

  9. Use the formula for the sum of the first nn terms of a geometric sequence, Sn=a(1−rn)1−rS_n = \frac{a(1-r^n)}{1-r}.

    We are given Sn>31S_n > 31.

    16(1−(12)n)1−12>31\frac{16\left(1 - \left(\frac{1}{2}\right)^n\right)}{1 - \frac{1}{2}} > 31
  10. Simplify the inequality.

    16(1−(12)n)12>31\frac{16\left(1 - \left(\frac{1}{2}\right)^n\right)}{\frac{1}{2}} > 31 32(1−(12)n)>3132\left(1 - \left(\frac{1}{2}\right)^n\right) > 31 1−(12)n>31321 - \left(\frac{1}{2}\right)^n > \frac{31}{32} 1−3132>(12)n1 - \frac{31}{32} > \left(\frac{1}{2}\right)^n 132>(12)n\frac{1}{32} > \left(\frac{1}{2}\right)^n
  11. Express both sides with the same base.

    (12)5>(12)n\left(\frac{1}{2}\right)^5 > \left(\frac{1}{2}\right)^n

    Since the base 12\frac{1}{2} is between 0 and 1, the inequality sign flips when comparing the exponents.

    5<n5 < n

    Since nn represents the number of terms, it must be an integer.
    The possible values of nn are n>5, where n∈Z\boxed{n > 5, \text{ where } n \in \mathbb{Z}}.

    2.2.3 Calculate the sum to infinity of this sequence.

  12. Use the formula for the sum to infinity of a geometric sequence, S∞=a1−rS_\infty = \frac{a}{1-r}.

    S∞=161−12S_\infty = \frac{16}{1 - \frac{1}{2}}
  13. Calculate the sum.

    S∞=1612S_\infty = \frac{16}{\frac{1}{2}} S∞=16×2S_\infty = 16 \times 2 S∞=32S_\infty = 32

    The sum to infinity of this sequence is 32\boxed{32}.

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