在ｓｃｈｅｍｅ里面，把数字平方，会出这样的结果：
比如说，平方１．２，结果是1.2100000000000002。
平方３．４，结果是11.559999999999999。
以前在ｊａｖａ里面也遇到过这种情形，为什么算出来是这样的结果呢，
明明是个准确值。。。

in IEEE standard, the double precision number 1.2 is represented by 0x3ff3333333333333 (8byte), and you have a round-off error.

for more details:
http://en.wikipedia.org/wiki/IEEE_754

"The numeric type is further divided into a numerical tower, with subtypes complex, real, rational and integer. (Note that these subtypes are not disjoint; in fact each type is a subset of the previous one). While it is not required that a Scheme implementation support the entire numerical tower, most implementations do. In addition to these traditional properties, Scheme numbers may have the property of "exactness". Integers and rational numbers are exact. An arithmetic operation involving numbers one or more of which is inexact has an inexact result.[1]"

In other words, some implementation of real numbers of scheme is imprecise.
Besides, I don't think Java has such problem.