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Ex 8.2, 7
Last updated at June 9, 2026 by Teachoo
Class 9
Chapter 8 - Predicting What Comes Next: Exploring Sequences & Progress
- Definition
- Explicit Rule for a Sequence
- Recursive Rule for a Sequence
- Virahānka–Fibonacci sequence
- Exercise Set 8.1
- Arithmetic Progressions
- Visualising an AP
- Sum of the First n Natural Numbers
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Exercise Set 8.2
- Geometric Progressions
- Fractals as a GP Sequence
- Visualising a GP
- Exercise Set 8.3
- End-of-Chapter Exercises
Transcript
Ex 8.2, 7 A child arranges marbles in rows so that the first row has 1 marble, the second has 2 marbles, the third has 3, and so on up to 25 rows. How many marbles does the child use in all? Our sequence of marbles are 1, 2, 3, 4, 5, …. 25 rows We see that the difference between two terms is constant So, this is an AP Now, First term = a = 1 Common difference = d = 2 – 1 = 1 Number of terms = n = 25 We need to find How many marbles does the child use in all So, we need to find Sn Now, Sn = 𝑛/2[2𝑎+(𝑛−1)𝑑] Putting a = 1, d = 1, n = 25 = 𝟐𝟓/𝟐[𝟐 × 𝟏+(𝟐𝟓−𝟏) × 𝟏] = 25/2[2+24 × 1] = 25/2[2+24] = 25/2 × 26 = 25 × 13 = 325 Thus, the child uses 325 marbles in all We need to find So, we need to find Sn Now, Sn = 𝑛/2[2𝑎+(𝑛−1)𝑑] Putting a = 1, d = 1, n = 25 = 𝟐𝟓/𝟐[𝟐 × 𝟏+(𝟐𝟓−𝟏) × 𝟏] = 25/2[2+24 × 1] = 25/2[2+24] = 25/2 × 26 = 25 × 13 = 325 Thus, the child uses 325 marbles in all
