$\textbf{Exercise.}\$
Let $a,b,c > 0$ with $a+b+c=1.\$
Show that
[ \frac{1}{a}+\frac{1}{b}+\frac{1}{c} \leq 3+2\cdot \frac{a^3+b^3+c^3}{abc} ]
$\textbf{Proof.}\$
WLOG
[1 = a+b+c > a+b > a > 0 \implies a,b,c \in (0,1) ]
Let
[x = \frac{1}{a}, \quad y = \frac{1}{b}, \quad z = \frac{1}{c} ]
Then I get
[ \frac{1}{x} + \frac{1}{y} + \frac{1}{z} = 1 \implies x,y,z > 1 ]
and thus prove
[ x+y+z \leq 3 + 2\cdot \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3}{\frac{1}{xyz}} ]
I will use the $\textbf{generalized mean or power mean}$ by Hölder.
[ \boxed{ \left ( \frac{1}{n} \sum_{i=1}^n x_i^{\beta} \right )^{\frac{1}{\beta}} \leq \left ( \frac{1}{n} \sum_{i=1}^n x_i^{\alpha} \right )^{\frac{1}{\alpha}} \quad \forall \alpha, \beta \in \mathbb{R}\setminus {0} \text{ where } \alpha > \beta } ]
getting the following
\begin{align*}
&\quad \frac{x+y+z}{3} \leq \left ( \frac{ \left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3 }{3} \right )^{\frac{1}{3}} \
&\implies \left ( \frac{x+y+z}{3} \right )^3 \leq \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3 }{3} \
&\implies \textcolor{cyan}{x+y+z \leq} \left ( x+y+z \right )^3 \leq \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3 }
{3^2}
\end{align*}
\begin{align*}
\implies x+y+z &\leq \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3 }
{3^2} \
&\leq \left ( \left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3 \right ) \cdot xyz \
&= \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3}{\frac{1}{xyz}} \
&\leq 2\cdot \frac{\left ( \frac{1}{x} \right )^3 + \left ( \frac{1}{y} \right )^3 + \left ( \frac{1}{z} \right )^3}{\frac{1}{xyz}} + 3\
\end{align*}