Let , , be positive real numbers such that . Find the minimum value of the expression
Solution
\begin{align*}
A &= \frac{2 - a^3}{a} + \frac{2 - b^3}{b} + \frac{2 - c^3}{c} = 2\left(\frac{1}{a} + \frac{1}{b} + \frac{1}{c}\right) - a^2 - b^2 - c^2 \\
&= 2\frac{ab + bc + ca}{abc} - (a^2 + b^2 + c^2) \\
&= 2\frac{ab + bc + ca}{abc} - \left((a + b + c)^2 - 2(ab + bc + ca)\right) \\
&= 2\frac{ab + bc + ca}{abc} - (9 - 2(ab + bc + ca)) \\
&= 2\frac{ab + bc + ca}{abc} + 2(ab + bc + ca) - 9 \\
&= 2(ab + bc + ca)\left(\frac{1}{abc} + 1\right) - 9
\end{align*}
Recall now the well-known inequality and set , , , to obtain where we have used . By taking the square roots on both sides of the last one we obtain:
Also by using AM-GM inequality we get that
Multiplication of (1) and (2) gives
So and the equality holds if and only if , so the minimum value is .