I've found in many exercises where I'm asked to show that $f(n)=\Theta(g(n))$ where the two functions are of the same order of magnitude I have difficulty finding a constant $c$ and a value $n_0$ for the lower bound. I'm using Corman's definition of $\Theta$:
$$\exists c_1,c_2>0\in\mathbb{R}:\forall n\geq n_0: 0 \leq c_1 g(n)\leq f(n)\leq c_2 g(n)$$
Showing the upper bound usually doesn't give me too much trouble, but for the lower bound I allot of times find myself using limits. And even though I'm getting the right answers, I'm a bit worried that my method isn't very rigorous and that maybe I'm doing a bit of hand waving in the process.
For example, problem 2.17 from Skiena's Algorithm Design Manual:
Show that for any $a,b\in \mathbb{R}: b>0$ that $(n+a)^b = \Theta(n^b)$
In this case I used limits to help find both constants.
For the upper limit I decided to look for some $c$ such that $(n+a)^b \leq c^bn^b$. So taking the $b$th root of each side and dividing by $n$ I have $\frac{n+a}{n}\leq c$ which gives me $1 + \frac{a}{n} \leq c$. For any $a\in\mathbb{R}$, $\lim_{n\to\infty }1+\frac{a}{n}=1$. If I pick $n_0>|a|$, then for $a<0$ the expression approaches 1 from the left starting arbitrarily close to $0$. If $a>0$ then the expression approaches 1 from the right starting arbitrarily close to 2. So choosing $c=2$ will satisfy the inequality and we have $c_2=2^b$.
Now for the lower bound. I'm looking at the same expression except with the inequality pointing the other way. In this case I'm trying to find $n_0$ and $c$ such that $c\leq 1+\frac{a}{n}$. The value of $n_0$ has to be greater than $|a|$ because otherwise we would have $c\leq 0$ which isn't allowed. This puts us in the same range of values between $0$ and $2$ approaching 1 from each side. So I choose any $c,n_0$ such that $n_0>|a|$ and $0 < c\leq 1-|\frac{a}{n_0}|$. So I could choose $n_0=3|a|$ and $c=\frac{2}{3}$.
Thus we have $0 < (\frac{2}{3})^bn^b \leq (n+a)^b \leq 2^bn^b$ for any $n \geq 3|a|$.
Is there an easier way to do this?
Normally when looking for upper limit constants where the two functions are of the same magnitude I simply eliminate negative lower order terms and change positive ones into multiples of the highest order term such as :
$$3n^2+15n-5\leq 3n^2+15n^2=18n^2$$
But when looking for the constant for the lower bound I find myself typically resorting to looking at limits. Is there any kind of short cut to finding the lower bound constant like there is for the upper bound constant?