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Tuesday, August 19, 2014

The Geometric Mean is just the Arithmetic Mean of the Exponents

This one is going to be brief, but I find it interesting. A buddy of mine a few months back sent me a text about this realization: The geometric mean is just the arithmetic mean of the exponents! I'll explain in a second but just a quick background.

An arithmetic mean is what we commonly refer to as the average: add all the elements up and divide by the number of elements. \overline{x}=\frac{x_1+x_2+\cdots+x_n}{n} Alternatively, the we have the geometric mean. Except the geometric mean is the multiplicative analog to the arithmetic mean. The geometric mean is the nth root of the product of the elements. \overline{y}'=\left(y_1y_2\cdots y_n\right)^{\frac{1}{n}} The connection is as follows. Each number y_i (assuming they are all positive) can be expressed as y_i=a^{b_i} for some a,b\in\mathbb{R}. Therefore we have the following \begin{align*} \overline{y}'&=\left(y_1y_2\cdots y_n\right)^{\frac{1}{n}}\\ &=\left(a^{b_1}a^{b_2}a^{b_2}\cdots a^{b_n}\right)^{\frac{1}{n}}\\ &=\left(a^{b_1+b_2+\cdots+b_n}\right)^{\frac{1}{n}}\\ &=a^{(b_1+b_2+\cdots+b_n)/n} \end{align*} which states that if each y_i can be expressed as y_i=a^{b_i} for some a\in\mathbb{R} and b_i\in\mathbb{R} then the geometric mean of all y_i is the base, a, raised to the arithmetic mean of all exponents b_i. \overline{y}'=a^{\overline{b}} Pretty interesting, eh?

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