The first five terms of a sequence are given below. 3278111815321950\frac{3}{2}\quad \frac{7}{8}\quad \frac{11}{18}\quad \frac{15}{32}\quad \frac{19}{50}

  1. (a)

    Write down the next two terms.

    Answer ___ , ___

  2. (b)

    The kkth term is 47288\dfrac{47}{288}. Find kk.

    Answer kk = ___

  3. (c)

    Find an expression, in terms of nn, for the nnth term.

    Answer ___

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Worked solution

9(a)

1. Identify and extend the numerator pattern.

The numerators are
3, 7, 11, 15, 193,\ 7,\ 11,\ 15,\ 19
They increase by 44 each time, so the next two numerators are
19+4=23,23+4=27.19+4=23,\qquad 23+4=27.

2. Identify and extend the denominator pattern and write the next two terms.

The denominators are
2, 8, 18, 32, 502,\ 8,\ 18,\ 32,\ 50
These are
2(12), 2(22), 2(32), 2(42), 2(52).2(1^2),\ 2(2^2),\ 2(3^2),\ 2(4^2),\ 2(5^2).
So the next two denominators are
2(62)=72,2(72)=98.2(6^2)=72,\qquad 2(7^2)=98.
Hence the next two terms are
2372,2798.\frac{23}{72},\qquad \frac{27}{98}.

Answer

2372, 2798\dfrac{23}{72},\ \dfrac{27}{98}

9(b)

1. Match the given numerator and denominator to the sequence pattern.

The numerators are 3,7,11,15,19,…3,7,11,15,19,\ldots, increasing by 44 each time.

So the numerator of the nnth term is 4n−14n-1.

For the given term 47288\dfrac{47}{288},
4k−1=474k-1=47
4k=484k=48
k=12k=12

Check denominator pattern: 2,8,18,32,50,…=2n22,8,18,32,50,\ldots=2n^2.

When k=12k=12, denominator =2(12)2=288=2(12)^2=288, so this matches.

Answer

k=12k=12

9(c)

1. Find the numerator pattern in terms of $n$.

The numerators are
3, 7, 11, 15, 19,…3,\ 7,\ 11,\ 15,\ 19,\ldots

This is an arithmetic sequence increasing by 44 each time.

For the nnth term, the numerator is
3+4(n−1)=4n−1.3+4(n-1)=4n-1.

2. Find the denominator pattern in terms of $n$ and form the nth term.

The denominators are
2, 8, 18, 32, 50,…2,\ 8,\ 18,\ 32,\ 50,\ldots

These can be written as
2(12), 2(22), 2(32), 2(42), 2(52),…2(1^2),\ 2(2^2),\ 2(3^2),\ 2(4^2),\ 2(5^2),\ldots

So the denominator of the nnth term is 2n22n^2.

Hence the nnth term is
4n−12n2.\frac{4n-1}{2n^2}.

Answer

4n−12n2\dfrac{4n-1}{2n^2}

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