你不是喜欢用wiki吗？自己学去。

这是关于算术的：
http://en.wikipedia.org/wiki/Arithmetic
这是关于数论的：
http://en.wikipedia.org/wiki/Number_theory

看出区别了没有？按你的逻辑，两个条目合并算了。后者还有
一段专门说arithmetic的用法的，别因为一个词有两种用法就
以为指的是同一个东西，当然，对你这种智力的人来说，这个
要求可能太高了。
The term "arithmetic" is also used to refer to number theory. This is a somewhat older term, which is no longer as popular as it once was. Number theory used to be called the higher arithmetic, but this too is dropping out of use. Nevertheless, it still shows up in the names of mathematical fields (arithmetic functions, arithmetic of elliptic curves, fundamental theorem of arithmetic). This sense of the term arithmetic should not be confused either with elementary arithmetic, or with the branch of logic which studies Peano arithmetic as a formal system. Mathematicians working in the field of number theory are called number theorists.