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9-15.

Ying has a different method for summing an infinite series. Here are her steps:

  1. Let the sum.

  2. Separate the first term from the rest of the series.

  3. Factor the rest of the terms so that the remaining factor is .

  4. Substitute for the other factor.

  5. Solve for .

Apply Ying's method to find in the following problem. The first three steps are done for you. Copy them first, then complete the solution.