.The
real-number system includes such numbers as –27,-2,2/3,… It is worthy of note
that positive numbers, 1/2, 1, for examples, are sometimes expressed as +(1/2),
+1. The plus sign, “+”, used here does not express the operation of addition, but
is rather part of the symbolism for the numbers themselves. Similarly, the
minus sign, “-“, used in expressing such numbers as -(1/2), -1, is part of thesymbolism for these numbers.
亲,真是太感谢啦 ,还有3大段,你方便的话也翻译了吧,不甚感激!!!http://zhidao.baidu.com/question/513011640.html?quesup2