2025 RI Promo Q9

2025 RI Promo Q9

Junior College 1
10 marks
  1. It is given that \(\sum_{r=1}^{n} \frac{2}{r(r+1)(r+2)}=\frac{1}{2}-\frac{1}{(n+1)(n+2)}\).
    1. State the sum to infinity of the series.[1]
    2. Find the sum of the first \(n\) terms of the series \(\frac{2}{17\times18\times19}+\frac{2}{18\times19\times20}+\frac{2}{19\times20\times21}+\cdots.\)[2]
    3. Find \(\sum_{r=2}^{n} \frac{1}{r(r^{2}-1)}\).[3]
  2. The sequence \(u_1, u_2, u_3, \ldots\) is defined by \(u_1=3\), \(u_{n+1}=2-\frac{4}{u_n}\), \(n\geq 1\).
    1. Find the values of \(u_2\), \(u_3\), \(u_4\) and \(u_{100}\).[2]
    2. Hence find \(\sum_{r=1}^{3n-2} u_r\) in terms of \(n\), simplifying your answer.[2]

Solution:

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Answer:(a)(i) \(\dfrac12\) (a)(ii) \(\dfrac1{306}-\dfrac1{(n+17)(n+18)}\) (a)(iii) \(\dfrac14-\dfrac1{2n(n+1)}\) (b)(i) \(u_2=\dfrac23,u_3=-4,u_4=3,u_{100}=3\) (b)(ii) \(\dfrac{10-n}{3}\)

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