$f ( x ) = x ^ { 3 } + x ^ { 2 } + c x + d$, where $c$ and $d$ are real numbers. Prove that if $c > \frac { 1 } { 3 }$, then $f$ has exactly one real root.
$f ( x ) = x ^ { 3 } + x ^ { 2 } + c x + d$, where $c$ and $d$ are real numbers. Prove that if $c > \frac { 1 } { 3 }$, then $f$ has exactly one real root.