Q.Find the geometric mean of the observations 2, 4 and 8, both directly and using logarithms.

Geometric Mean: The "Multiplicative Average"

Most students first meet the arithmetic mean — add up the numbers, divide by how many there are. That works when you want the additive centre of a set. But what if your data grows by multiplication, not addition? That is where the geometric mean comes in.

Intuition: Growth, not Sum

Imagine you invest ₹100. Year 1: it grows by 10% (multiply by 1.10). Year 2: it grows by 20% (multiply by 1.20). What is the average growth rate per year?

If you take the arithmetic mean of 10% and 20%, you get 15%. But check: ₹100 grown by 15% for two years gives $100 \times 1.15 \times 1.15 = ₹132.25$. The actual result is $100 \times 1.10 \times 1.20 = ₹132.00$. The arithmetic mean overestimates.

Why? Because growth compounds — each year's multiplier acts on the previous result, not on the original. The correct "average multiplier" is the one that, applied twice, gives the same final product. That is the geometric mean.

The Precise Statement

For a set of $n$ positive numbers $x_1, x_2, \dots, x_n$, the geometric mean $G$ is the $n$-th root of their product:

For two numbers $a$ and $b$: $G = \sqrt{ab}$

For three numbers $a, b, c$: $G = \sqrt[3]{abc}$

Why It Works (The "Why" Before the Formula)

Return to the investment example. You want a single growth rate $r$ such that:

Cancel the 100 on both sides:

So $1+r = \sqrt{1.10 \times 1.20} \approx 1.1489$, meaning $r \approx 14.89\%$. That is the geometric mean of the two multipliers.

Notice: you took the square root of the product of the multipliers. That is exactly the geometric mean formula.

Key Properties (Exam-Ready)

• Always ≤ arithmetic mean (for positive numbers). Equality only when all numbers are identical.
• Scale invariance: multiply every number by a constant $k$, and the geometric mean also multiplies by $k$. …