主题：考考你的文化素质（高难） -- 水西残梦

1 考虑(x+y)^2,易得xy+yx=0,x+x=0,char(R)=2,。所以R中只有一个元素0可以写成a+a的形式，R中所有元素都可以写成a+a+a的形式。

4 t=sqrt(2L/g), 3mg

5 All books are divisible into two kinds: the books of the hour, and the books of all time. Mark this distinction---it is not one of quality only. It is not merely the bad book that does not last, and the good one that does. It is a distinction of species.