# Functional equation

#### Alexmahone

##### Active member
Suppose $f(x)$ is continuous for all $x$ and $f(a+b)=f(a)+f(b)$ for all $a$ and $b$. Prove that $f(x)=Cx$, where $C=f(1)$.

I have shown that $f(x)=Cx$ for all rational numbers. How do I use the continuity of $f$ to show it is true for all $x$?

#### Krizalid

##### Active member
I have shown that $f(x)=Cx$ for all rational numbers.
Having this, remember that rationals are dense in $\mathbb R.$

#### Alexmahone

##### Active member
Having this, remember that rationals are dense in $\mathbb R.$
Intuitively, I can see that it must be true but I'm having trouble proving it.

#### Plato

##### Well-known member
MHB Math Helper
Intuitively, I can see that it must be true but I'm having trouble proving it.
Every real number is the limit of a sequence of rational numbers.
The function is continuous. What continuity and convergent sequences?

#### Alexmahone

##### Active member
Every real number is the limit of a sequence of rational numbers.
The function is continuous. What continuity and convergent sequences?
Got it. Thanks!

#### HallsofIvy

##### Well-known member
MHB Math Helper
By the way, if you do not include the requirement that the function be continuous, all f are either of the form f(x)= cx or the graph of y= f(x) is dense in the plane.