# Math Help - Compare

1. ## Compare

a and b being two strictly positive numbers , compare (a+b)/2 and √a√b

2. Originally Posted by lebanon
a and b being two strictly positive numbers , compare (a+b)/2 and √a√b
Consider $(\sqrt{a}-\sqrt{b})^2=(\sqrt{a})^2-2\sqrt{a}\sqrt{b}+(\sqrt{b})^2=a+b-2\sqrt{a}\sqrt{b}$.

Then divide by 2 to get $\displaystyle\frac{(\sqrt{a}-\sqrt{b})^2}{2}=\frac{a+b}{2}-\sqrt{a}\sqrt{b}$. But since $\displaystyle\frac{(\sqrt{a}-\sqrt{b})^2}{2}$ is nonnegative for any positive $a$ and $b$ (why?) it follows that $\displaystyle\frac{a+b}{2}-\sqrt{a}\sqrt{b}\geq0$.

Therefore, $\displaystyle\frac{a+b}{2}\geq\sqrt{a}\sqrt{b}$ for any positive numbers $a$ and $b$.

3. And it can further be proven by simple induction that:

$\displaystyle \left(\prod_{1\le i \le n}a_{i}\right)^{\frac{1}{n}} \le \frac{1}{n}\sum_{1\le i\le n}a_{i}$ for $a_{i} \in\mathbb{R}^{+}$ and $n\in\mathbb{N}$.