Harmonic mean of two numbers, 2pq/(p+q): write a relation or expression

1. A.
   1
2. B.
   2
3. C.
   $\frac{P + Q}{PQ}$
4. D.
   $\frac{PQ}{P + Q}$

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1. A.
   $\frac{2S}{P}$
2. B.
   $\frac{S}{(2P)}$
3. C.
   $\frac{2P}{S}$
4. D.
   $\frac{P}{(2S)}$

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1. A.
   $xy + xz - yz = 0$
2. B.
   $xy + xz - 2yz = 0$
3. C.
   $xy + xz + yz = 0$
4. D.
   $xy + xz - 4yz = 0$

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