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In an infinite geometric progression, the sum of all the even-positioned terms is 1 4 of the sum of the entire progression. If the first term of the progression is 4 , then the sum of the squares of all the terms of the progression is equal to

Options

  1. A256 15
  2. B18
  3. C24
  4. D8

Correct answer

B. 18

Step-by-step solution

Let the infinite geometric progression be a, ar, ar^2, ar^3, The sum of the entire progression is S_ total = a 1 - r . The sum of the even-positioned terms is S_ even = ar + ar^3 + ar^5 + = ar 1 - r^2 . Given that S_ even = 1 4 S_ total , we can write: S_ total = S_ odd + S_ even S_ odd = S_ total - 1 4 S_ total = 3 4 S_ total . Since each even-positioned term is r times the preceding odd-positioned term, we have S_ even = r S_ odd . r = S_ even S_ odd = 1 4 S_ total 3 4 S_ total = 1 3 . We are given the first term

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