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- PMT-101 Preparatory Course in General Mathematics (BPP)PMT-101
- Unit-6 Counting Without CountingUnit-6

Unit-6 Counting Without Counting
In this unit you will learn about Arithmetic Progression, Geometric Progression and other sequences.
Find the sum of the following GPs
i) 6 + 12 + · · · to 7 terms.
ii) 2 + 8 + 32 + · · · + 2048

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Learning Pundits Content Team
i)The ratio of the second term to the first term, 12/6 = 2, so the common ratio is 2. The sum to 7 terms of this GP is 6 × (2^6-1)/(2-1) = 378.
ii)The ratio of the second term to the first term is 8/4 = 4. Suppose 2048 is the nth term. We have to find the value of n. Then, 2 × 4^n-1 = 2048 or 4^(n-1) = 1024. Since 4^5 = 1024, we have n = 6. So, 2048 is the 6th term of the above GP. Again using the formula given , the sum is 2 x (4^6 -1)/(4-1) = 2 × 341 = 2730