（1）原式= 2a?a?2 (a+2)(a?2) = 1 a+2 ；（2）原式=[ x+2 x(x?2) ? x?1 (x?2)2 ]? x ?(x?4) = x2?4?x2+x x(x?2)2 ? x ?(x?4) = ?1 (x?2)2 ；x取4和2以外的任何数，代入计算正确即可．
