Q:

Asymptotic behavior of polynomials

The following question is taken from Introduction to Algorithms by CLRS, Chapter $3.$

Let 
  $$p(n) = \sum_{i=0}^d a_i n^i,$$
  where $a_d>0,$ be a degree-$d$ polynomial in $n$, and let $k$ be a constant.  Use the definitions of the asymptotic notations to prove the following properties. 
a. If $k\geq d,$ then $p(n)= O(n^k).$
b. If $k\leq d,$ then $p(n)= \Omega(n^k).$
c. If $k =  d,$ then $p(n)= \Theta(n^k).$
d. If $k> d,$ then $p(n)= o(n^k).$
a. If $k< d,$ then $p(n)= \omega(n^k).$

My attempt:
From exercise $3.1$-$1$ in the same chapter, we have proven that for asymptotically nonnegative function $f(n)$ and $g(n),$
$$max(f(n),g(n)) = \Theta(f(n)+g(n)).$$
So we have 
$$\Theta(f(n)+g(n)) = \Theta(max(f(n),g(n))).$$
It follows that 
$$p(n) = \Theta(p(n)) = \Theta\left( \sum_{i=0}^d a_i n^i \right) = \Theta(n^d).$$
So all parts (a)-(e) follow.  
Is my attempt above correct? 

A:

You are using:
$\displaystyle 0<\lim\limits_{n\to a}\inf\left|\frac{\max(f(n),g(n))}{f(n)+g(n)}\right|\leq\lim\limits_{n\to a}\sup\left|\frac{\max(f(n),g(n))}{f(n)+g(n)}\right|<\infty~$ 
We get a problem, if $~f~$ and $~g~$ are polynomials and at least one is no constant: 
$\max(f(n),g(n))=~\infty~$
It's impossible to use your argumentation, it's better to use the definitions:  
a. $\displaystyle ~p(n)= O(n^k) :~~\lim\limits_{n\to\infty}\sup\left|\frac{p(n)}{n^k}\right|<\infty$
b. $\displaystyle ~p(n)= \Omega(n^k) :~~\lim\limits_{n\to\infty}\inf\left|\frac{p(n)}{n^k}\right|>0~$ 
c. $\displaystyle ~p(n)= \Theta(n^k) :~~0<\lim\limits_{n\to\infty}\inf\left|\frac{p(n)}{n^k}\right|\leq\lim\limits_{n\to\infty}\sup\left|\frac{p(n)}{n^k}\right|<\infty~$ , $~$it's $~$a.$~$ and $~$b. 
d. $\displaystyle ~p(n)= o(n^k) :~~\lim\limits_{n\to\infty}\left|\frac{p(n)}{n^k}\right|= 0$
e. $\displaystyle ~p(n)= \omega(n^k) :~~\lim\limits_{n\to\infty}\left|\frac{p(n)}{n^k}\right|= \infty$

