Prove that if $f:(0,\infty)→\mathbb{R}$ satisfying $f(xy)=f(x)+f(y)$, and if $f$ is continuous at $x=1$, then $f$ is continuous for $x>0$.
I let $x=1$ and I find that $f(x)=f(x)+f(1)$ which implies that $f(1)=0$. So, $\lim_{x\to1}f(x)=0$, but how can I use this to prove continuity of $f$ for every $x \in \mathbb R$?
Any help would appreciated. Thanks
Answer
Give $x_0>0$, $$f(x)-f(x_0)=f\left(x_0\cdot\frac{x}{x_0}\right)-f(x_0)=f\left(\frac{x}{x_0}\right),$$ by $f$ is continuous at $x=1$, when $x\to x_0$, $\frac{x}{x_0}\to1$, then $$\lim\limits_{x\to x_0}f(x)=f(x_0).$$
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