Analysis calculus proof kick start question

[gaborfk]

1. The problem statement, all variables and given/known data
Prove: If f is defined on \mathbb{R} and continuous at x=0, and if f(x_{1}+x_{2})=f(x_{1})+f(x_{2}) \forall x_{1},x_{2} \in\mathbb{R}, then f is continous at all x\in\mathbb{R}.


2. Relevant equations

None

3. The attempt at a solution

Need a pointer to get started. Cannot wrap my head around it. I understand that I need to prove that the sum of two continuous functions is continous also.

[ircdan]

there is only one function here, f , it has the property that f(x + y) = f(x) + f(y) for all x, y


hint, show f(0) = 0

[gaborfk]

I know that the function is continuous at x=0. So how does showing it is continuous at zero help with showing the function with the property f(x_{1}+x_{2})=f(x_{1})+f(x_{2}) is continuous?

Thank you

[HallsofIvy]

f is continuous at x= a if and only if
[tex]\lim_{x\rightarrow a}f(x)= f(a)[/itex].

If h= x- a, then x= a+ h and h goes to 0 as x goes to a: that becomes
\lim_{h\rightarrow 0}f(x+a)= f(a).

[tiny-tim]

Hi gaborfk! :smile:

Hint: f(a + epsilon) = f(a) + f(epsilon) :smile: