Find the sum of the G.P. :

4/5 , 4/25 , 4/125 , 4/625 ,... to n terms.

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UP Police SI (दरोगा) Official PYP (Held On: 21st Nov 2021 shift 3)
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  1. 2/5(1-(1/5n)) 
  2. 5/4(1-(1/5n))
  3. (1-(1/5n))
  4. 4/5(1-(1/5n)) 

Answer (Detailed Solution Below)

Option 3 : (1-(1/5n))
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Detailed Solution

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Given:

Series is 4/5 , 4/25 , 4/125 , 4/625 ,... to n term

Formula used:

Sum = a (1 - rn) / (1 - r )

Where a = first term & r = common difference 

Calculation:

Common difference (r) = (4/25) ÷ (4/5)

⇒ (1/5)

Putting the value of r in the above formula we get:

Sum = \(\frac{4}{5}\)(1 - \(\frac{1}{5}\)n) / 1 - \(\frac{1}{5}\)

⇒ \(\frac{4}{5}\)(1- (\(\frac{1}{5}\))n) / \(\frac{4}{5}\)

⇒ (1 - (\(\frac{1}{5}\))n)

Hence the correct answer is "​​​(1 - (​\(\frac{1}{5}\))n)".

 

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