Out of curiosity, how does one get from 1 / ( 1/2 (1/P + 1/R) ) to 2 (PR / P+R) ?

I think that’s just a bit of algebra.

Yes, I believe.

It is pretty easy to get from:

1 / ( 1/2 (1/P + 1/R) )

to

2 / (1/P + 1/R)

But from there to

2PR / (P+R) seems just incomprehensible.

Hello @Lassi_Elo, we can just focus on this → 1/P + 1/R and the next step would be to find the common denominator which is PR. Can you continue from here?

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Thanks, I got it! I forgot that I can just focus on one part of the equation at a time.