Find the sum of the G.P.:
5/11, 5/121, 5/1331, 5/14641, ... to n terms.

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UP Police SI (दरोगा) Official PYP (Held On: 27 Nov 2021 Shift 3)
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  1. 1/5(1-(1/11)n)
  2. 3/10(1-(1/11)n)
  3. 3/7(1-(1/7)n)
  4. 1/2(1-(1/11)n)

Answer (Detailed Solution Below)

Option 4 : 1/2(1-(1/11)n)
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UP Police SI (दरोगा) Official PYP (Held On: 2 Dec 2021 Shift 1)
160 Qs. 400 Marks 120 Mins

Detailed Solution

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Given:

G.P. = 5/11, 5/121, 5/1331, 5/14641, ... up to n terms

Formula used:

G.P. sum to n terms = a × (1 - rn) / (1 - r)

First term (a) = 5/11

Common ratio (r) = (5/121) ÷ (5/11) = 1/11

Calculations:

Sum = (5/11) × [1 - (1/11)n] / (1 - 1/11)

⇒ Sum = (5/11) × [1 - (1/11)n] / (10/11)

⇒ Sum = (5/11) × (11/10) × [1 - (1/11)n]

⇒ Sum = (5/10) × [1 - (1/11)n] = (1/2) × [1 - (1/11)n]

∴ Sum to n terms = (1/2) × [1 - (1/11)n]

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