原式=x-1x(x+1)•(x+1)(x-1)(x-1)2-1x
=1x-1x
=0．
Yesterday was the twins’ birthday, ________ presented a gift.
A . each of them B each of whom C both of them D both of whom
按表观密度不同，混凝土分（）。
A.重混凝土
B.轻混凝土
C.密混凝土
D.普通混凝土
E.防辐射混凝土
