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Question

The sum of an infinite geometric series is 3. When the common ratio of the series is doubled, then the sum becomes 5. The first term of the series is
  1. 87
  2. 157
  3. 177
  4. 27


Solution

The correct option is B 157
Let the first term be a and the common ratio be r,
a1r=3.....(i)
a12r=5.....(ii)
Dividing the two equation,
12r1r=35510r=33rr=27
Using equation (i),
a127=3a=157

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