Q.For positive real numbers $a, b$ and $c$ if $a + b + c = 18$, find the maximum value of $abc$.

The Intuition: Why the Average Isn't Always the Middle

Imagine you have two numbers: 2 and 8. Their ordinary arithmetic mean (AM) is $(2+8)/2 = 5$. That feels natural — the number halfway between them on a number line.

Now think about the geometric mean (GM). For 2 and 8, it's $\sqrt{2 \times 8} = \sqrt{16} = 4$. That's smaller than 5. Why?

The geometric mean cares about multiplicative spacing, not additive spacing. On a number line, 2 and 8 are 6 units apart, so the arithmetic mean sits right in the middle. But if you think in terms of ratios: 2 multiplied by 4 gives 8. The geometric mean is the number that, when multiplied by itself, gives the same product — it's the "multiplicative middle." And that middle is always pulled toward the smaller number when the numbers are unequal.

The Precise Statement

For any two positive real numbers $a$ and $b$:

with equality if and only if $a = b$.

Where:

This inequality is not just a curiosity — it's a fundamental result that holds for any set of positive numbers, not just two. For $n$ positive numbers $x_1, x_2, \dots, x_n$:

Why It's True: A Simple Proof

Start with any two positive numbers $a$ and $b$. Consider $(\sqrt{a} - \sqrt{b})^2$.

Since a square is always non-negative:

Expand:

Rearrange:

Divide by 2:

That's it. The equality case is when $\sqrt{a} - \sqrt{b} = 0$, i.e., $a = b$.

A Concrete Example

Take $a = 4$, $b = 9$.

• AM = $(4+9)/2 = 6.5$ …