# Check my proof of the quotient rule of limits for functions.

Given that: $$\lim_{x \to c}f(x)=L_1$$which means(1): $$\forall\epsilon_1>0, \exists \delta_1>0:|x-c|<\delta_1\implies|f(x)-L_1|<\epsilon_1$$ and $$\lim_{x \to c}g(x)=L_2, \space \text{where}\space g(x), \space L_2\ne0$$ which means(2): $$\forall\epsilon_2>0, \exists \delta_2>0:|x-c|<\delta_2\implies|g(x)-L_2|<\epsilon_2$$

I want to prove: $$\lim_{x \to c}\frac{f(x)}{g(x)}=\frac{\lim_{x \to c}f(x)}{\lim_{x \to c}g(x)}=\frac{L_1}{L_2}$$

which means showing: $$\forall \epsilon>0,\exists \delta>0:|x-c|<\delta\implies|\frac{f(x)}{g(x)}-\frac{L_1}{L_2}|<\epsilon$$

First, let: $$\delta=\min(\delta_1,\delta_2),\space \epsilon=2a, \space \epsilon_1=ak,\space \epsilon_2=an$$where $$\frac{1}{k}>\frac{1}{|L_2|} \space \text{and} \space \frac{1}{n}>|\frac{f(x)}{g(x)L_2}|$$ $k,n>0$ from how we defined them.

Substituting $\epsilon_1$ in (1) gives us:$$|f(x)-L_1|<\epsilon_1=ak$$ which implies(3): $$\frac{1}{|L_2|}\cdot |f(x)-L_1|<a$$

The same procedure is done for (2), which gives: $$|g(x)-L_2|<\epsilon_2=an$$ which implies(4): $$|\frac{f(x)}{g(x)L_2}|\cdot|g(x)-L_2|<a$$

Adding (3) and (4) gives:$$\frac{1}{|L_2|}\cdot |f(x)-L_1|+|\frac{f(x)}{g(x)L_2}|\cdot|g(x)-L_2|=\frac{1}{|L_2|}\cdot |f(x)-L_1|+|\frac{f(x)}{g(x)L_2}|\cdot|L_2-g(x)|\\=|\frac{f(x)-L_1}{L_2}|+|\frac{f(x)}{g(x)}-\frac{f(x)}{L_2}|\ge |\frac{f(x)-L_1}{L_2}+\frac{f(x)}{g(x)}-\frac{f(x)}{L_2}|$$

Finally, $$|\frac{f(x)-L_1}{L_2}+\frac{f(x)}{g(x)}-\frac{f(x)}{L_2}|=|-\frac{L_1}{L_2}+\frac{f(x)}{g(x)}|=|\frac{f(x)}{g(x)}-\frac{L_1}{L_2}|<2a=\epsilon$$ and $|\frac{f(x)}{g(x)}-\frac{L_1}{L_2}|$ is indeed less than $\epsilon$. We have proved what we wanted.

• You should add a little more explanation about choosing $n$ (essentially this means that $f/g$ is bounded and this is where the fact that $L_{2}\neq 0$ is needed). Oct 22 '17 at 8:35
• The reason why I chose $\frac{1}{n}>|\frac{f(x)}{g(x)L_2}|$ is because it's more "succinct" than $n<|\frac{g(x)L_2}{f(x)}|$ which is what I originally chose during my derivation of the proof. If I had gone with the latter choice, I would need to make $f(x) \ne 0$. Oct 22 '17 at 8:54
• My point was that you should explain why such an $n$ exists. This requires you to show that $|f(x) |<|L_1|+1$ and $|g(x) |>|L_2|/2$. Oct 22 '17 at 9:02
• I'm clueless on how did you arrive to that conclusion. I interpreted that as finding a value for $n$ such that it satisfies $\frac{1}{n}>|\frac{f(x)}{g(x)L_2}|$ It took me some time to figure out but I'm lost. How should I start? Oct 22 '17 at 9:54
• $n$ should be constant and not depend on $x$. Oct 22 '17 at 10:22

Your proof is fine. If you have the limit theorem for products, you can instead apply that to $f(x)$ and $1/g(x)$ rather than having to go through all of that (but you would have to also prove $\lim 1/g(x) = 1/\lim g(x)$ which is essentially the difficulty).
The proof is correct except for a minor step. You propose that there is a number $n>0$ such that $|f(x) /g(x) L_{2}|<1/n$. This is true but not obvious / self-evident. Note that by choosing $\epsilon=1$ in the limit definition for $f$ we have a $\delta_{3}>0$ such that $$0<|x-a|<\delta_{3}\implies |f(x) - L_{1}|<1\implies |f(x) |<|L_{1}|+1$$ and choosing $\epsilon=|L_{2}|/2>0$ in the limit definition of $g$ we have a $\delta_{4}>0$ such that $$0<|x-a|<\delta_{4}\implies|g(x)-L_{2}|<|L_{2}|/2\implies |g(x) |>|L_{2}|/2$$ Therefore $$0<|x-a|<\min(\delta_{3},\delta_{4})\implies \left|\frac{f(x)} {g(x) L_{2}}\right|<\frac{2(|L_{1}|+1)}{|L_{2}|^{2}}=\frac{1}{n}\text{ (say)}$$ Now on the basis of $\epsilon, n, k$ choose your $\delta_{1},\delta_{2}$ and keep $\delta$ as minimum of all the deltas.
• What led you to find the values of $\epsilon=1$ and $\epsilon=\frac{|L_2|}{2}$? Is it just trial and error? Genuinely interested in the process;All my approaches so far to find a suitable $\epsilon$ value involves me going backwards from the result I want to prove. Oct 22 '17 at 10:41
• @DanielMcFluffy: as I said earlier the idea is to bound the ratio $f/g$ and this requires an upper bound on $f$ and lower bound on $g$. The choice of epsilon in both cases is almost arbitrary. But we need a constant as bound so I chose specific real number $1$ (you can choose any number like $0.001$ or $\sqrt{2}$ as long as it is positive. For $g$ we need $\epsilon$ to be less than $|L_{2}|$. Understand that limit definition is not an exercise in symbolic manipulation and formal logic, rather it is the art of bounding expressions suitably. Cont'd. Oct 22 '17 at 11:07
• @DanielMcFluffy: to bound $f/g$ note that $f$ has a limit so $f$ is bounded and $g$ has a non-zero limit so it is bounded away from zero (ie we can ensure that values of $g$ are near to $L_{2}$ and farther from $0$). Oct 22 '17 at 11:09