harmonic mean

x1, x2, x3, ..., xn are average speeds for a fixed distance. What's the average speed of the trip?

Harmonic mean of those speeds = n/{(1/x1)+(1/x2)+ ... + (1/xn)}

For n =2, 2/{(1/x1)+(1/x2)} = 2x1*x2/(x1+x2)


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