Re: What a great site!
The equation for parallel resistances (and impedances) is thus:
1/R1 + 1/R2 = X, 1/X = Rtotal
I'm no math whiz so forgive if the formula isn't expressed correctly. But for the two impedances you mentioned, 4 & 2:
For more than two parallel resistances, just keep repeating the 1/X + string for each and then divide 1 by the total.
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