Select Page

problem: For ${x > 0}$, show that ${\displaystyle{\frac{x^n - 1}{x - 1}}{\ge}{n{x^{\frac{n - 1}{2}}}}}$, where ${n}$ is a positive integer.

solution: ${\displaystyle{\frac{x^n - 1}{x - 1}}{\ge}{n{x^{\frac{n - 1}{2}}}}}$

${\Leftrightarrow}$ ${\displaystyle{\frac{(x - 1)(x^{n - 1} + x^{n - 2} + ......... + x + 1)}{x - 1}}}$ ${> n x^{\frac{n - 1}{2}}}$

${\Leftrightarrow}$ ${\displaystyle{\frac{x^{n - 1} + x^{n - 2} + ......... + x^1 + x^0}{n}}}$ ${> x^{\frac{n - 1}{2}}} (\dagger)$

Now to prove $$( \dagger)$$ we observe:

But  $\displaystyle{\frac{x^{n - 1} + x^{n - 2} + ......... + x^1 + x^0}{n} \\ > \{x^{n-1}\cdot x^{n-2} \cdots x^0 \}^{\frac{1}{n}} \\ = \{x^{(n-1) + \cdots 0} \}^{\frac{1}{n}} \\ =\{ x^{\frac{n(n-1)}{2}}\}^\frac{1}{n}} \\ = x^{\frac{(n-1)}{2}}$

Now this follows directly from AM-GM inequality.