Prove inequality $2e^x>x^3+x^2$ If $x \in \Bbb R$, show that
$$2e^x>x^3+x^2$$
This inequality is right, see

Own ideas: If $x\in \Bbb R$,
$$f(x)=2e^x-x^3-x^2$$
$$f'(x)=2e^x-3x^2-2x$$
$$f''(x)=2e^x-6x-2$$
$$f'''(x)=2e^x-6$$
$$f''''(x)=2e^x>0$$
Like the symbol cannot judge $f$ sign.
So how can we show this $$f(x)>0 \text{ for } x \in \Bbb R?$$

Consider that for all x in $\mathbb R$, $e^{x}$ > 0, so  $2e^{x}$ > 0 for all $\mathbb R$.So clearly for $x\leq 0$ in $\mathbb R$, the statement is true since $x^2 \geq 0$ and  $x^3 \leq 0$.  Since $|x^3| \geq x^2$ for x in $\mathbb R$, $x^3 + x^2 \leq 0$ for $x\leq 0$ in $\mathbb R$. 
Now consider where $x>0$ in $\mathbb R$. Let's use the exponential power series since we know it converges for all real numbers. Assuming x > 0,rearranging and substituting gives $e^x > \frac{1}{2} (x^3 + x^2)$ =  $\sum_{n=1}^{\infty} \frac{x^n}{n!} > \frac{1}{2} (x^3 + x^2)$
The question now is which side grows larger as $x\rightarrow +\infty$. Clearly both go to infinity in this case. So consider the following:  
$\frac{e^x} {\frac{1}{2} (x^3 + x^2)}$  
Now use L'Hospital's rule: 
$\frac{e^x} {\frac{3}{2} (x^2 + x)}$ 
Apply again: 
$\frac{e^x} {3x + 1}$
One last time. 
$\frac{e^x} {3}$
Clearly as $x\rightarrow +\infty$ then  e^x > 3. That completes the proof. Whew!     
