An Exercise on Sum to Infinity | Convergence & Applications

Synopsis

This article covers the essential skills of sum to infinity of geometric series – an advanced topic in DSE Paper 1 Section A(2) and Section B worth 4–6 marks. You will learn the convergence condition , the sum to infinity formula , how to convert recurring decimals to fractions using infinite series, and how to solve application problems such as bouncing balls, infinite processes, and financial models. The article includes step-by-step worked examples, DSE exam techniques, practice questions. These are high-level skills that can help you secure top marks.

Article

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 Sum to Infinity | Convergence & Applications

Exercise

Section A(2) & Section B Practice

Question 1 MC
Find the sum to infinity of \( 9 + 3 + 1 + \frac{1}{3} + \cdots \).
A. \( 12 \)     B. \( 13.5 \)     C. \( 15 \)     D. \( 18 \)

Question 2 MC
Which series does not converge?
A. \( 4 + 2 + 1 + 0.5 + \cdots \)     B. \( 1 - \frac{1}{2} + \frac{1}{4} - \frac{1}{8} + \cdots \)
C. \( 10 + 5 + 2.5 + 1.25 + \cdots \)     D. \( 2 + 6 + 18 + 54 + \cdots \)

Question 3 Short Answer
Convert \( 0.\overline{45} \) to a fraction.

Question 4 Short Answer
Find the sum to infinity of \( 27 + 9 + 3 + 1 + \cdots \).

Question 5 MC
A ball rebounds to \( \frac{2}{3} \) of its previous height. If dropped from 81 m, total distance traveled is:
A. \( 243 \) m     B. \( 405 \) m     C. \( 324 \) m     D. \( 486 \) m

Question 6 Short Answer
Express \( 1.\overline{6} \) as a fraction.

Question 7 MC
The sum to infinity of a GP is 24 and the first term is 12. Find the common ratio.
A. \( \frac{1}{2} \)     B. \( \frac{1}{3} \)     C. \( \frac{2}{3} \)     D. \( \frac{3}{4} \)

Question 8 Short Answer
A person walks 40 m east, then 20 m west, then 10 m east, and so on. Find the total distance traveled.

Question 9 Short Answer
The sum to infinity of a GP is twice its first term. Find the common ratio.

Question 10 Short Answer
Write \( 0.\overline{27} \) as a fraction and verify by long division.

Answer Key for Exercise

Q1 B (\( 13.5 \))
Q2 D (\( r=3 \))
Q3 \( \frac{5}{11} \)
Q4 \( 40.5 \) (or \( \frac{81}{2} \))
Q5 B (\( 405 \) m)
Q6 \( \frac{5}{3} \)
Q7 A (\( \frac{1}{2} \))
Q8 \( 80 \) m (net displacement \( \frac{40}{3} \) but total distance is sum of absolute values: \( 40 + 20 + 10 + \cdots = 80 \)) Wait, total distance traveled: east+west+... = \( 40 + 20 + 10 + 5 + \cdots = 80 \). So answer 80.
Q9 \( r = \frac{1}{2} \)
Q10 \( \frac{3}{11} \)
Common DSE Trap

When using \( S_\infty = \frac{a}{1-r} \), remember the condition \( |r| < 1 \). If \( r \) is not in this range, the sum does not exist. Also, for bouncing ball problems, remember to count both upward and downward distances appropriately.