MHB Check existence of limit with definition

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To check the existence of the limit $\lim_{x\to 0}\frac{x}{x}$ using the epsilon-delta definition, it's necessary to establish that for every $\epsilon > 0$, there exists a $\delta > 0$ such that if $0 < |x| < \delta$, then $|f(x) - 1| < \epsilon$. The expression simplifies to $|f(x) - 1| = \left|\frac{x}{x} - 1\right| = 0$ for all $x \neq 0$. This indicates that the limit exists and equals 1 as $x$ approaches 0. The discussion confirms that the limit is indeed 1, validating the use of the epsilon-delta definition.
mathmari
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Hey! :o

I want to check the existence of the limit $\lim_{x\to 0}\frac{x}{x} $ using the definition.

For that do we use the epsilon delta definition?

If yes, I have done the following:

Let $\epsilon>0$. We want to show that there is a $\delta>0$ s.t. if $0<|x-0|<\delta$ then $|f(x)-1|<\epsilon$.

We have that $\left |f(x)-1\right |=\left |\frac{x}{x}-1\right |=\left |\frac{x-1}{x}\right |=\frac{|x-1|}{|x|}$.

How can we continue? (Wondering)
 
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Hey mathmari!

Since $0<|x-0|$, it follows that $x\ne 0$.
Therefore $\left|\frac xx -1\right|=0$, isn't it? (Wondering)
 
Klaas van Aarsen said:
Since $0<|x-0|$, it follows that $x\ne 0$.
Therefore $\left|\frac xx -1\right|=0$, isn't it? (Wondering)

Ohh yes! Thank you! (Blush)
 
There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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