Means - Arithmetic, Geometric, and Harmonic

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Multiple Choice

The harmonic mean of $\dfrac { a }{ 1-ab } and \dfrac { a }{ 1+ab }$ is:

  1. $a$
  2. $\dfrac { a }{ 1-{ a }^{ 2 }b^{ 2 } }$
  3. $\dfrac { 1 }{ 1-{ a }^{ 2 }b^{ 2 } }$
  4. $\dfrac { a }{ 1+{ a }^{ 2 }b^{ 2 } }$
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