Example 1 · level 1
Find the 21th term of the AP 17, 22, 27, …
Show solution
- a = 17, d = 5.
- t21 = 17 + (21 − 1) × 5 = 117.
Answer: 117
CA FOUNDATION: QUANTITATIVE APTITUDE · SEQUENCE AND SERIES
Sequences: AP and GP terms and sums
AP: tₙ = a + (n − 1)d, Sₙ = n/2 [2a + (n − 1)d]. GP: Sₙ = a(rⁿ − 1)/(r − 1) for r > 1.
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Worked examples
Example 2 · level 2
Find the sum of the first 20 terms of the AP 4, 6, 8, …
Show solution
- a = 4, d = 2, n = 20.
- Sₙ = 20/2 × [8 + 38] = 20/2 × 46 = 460.
Answer: 460
Example 3 · level 3
Gupta & Sons saves ₹4,000 in month 1 and multiplies the saving by 2 every month (₹4,000, ₹8,000, ₹16,000, …). Find the total saved in 9 months.
Show solution
- a = 4000, r = 2, n = 9.
- Sₙ = 4000 × (512 − 1) ÷ 1
- = ₹20,44,000.
Answer: ₹20,44,000