(4) 解:原式=1/2x(4a²-4a+1)=1/2x(2a-1)²
(5)解:原式=1/4【(a+b)²+2(a+b)+1】=1/4(a+b+1)²
9﹙x+y+z﹚²-﹙x-y-z﹚²=[3﹙x+y+z﹚+﹙x-y-z﹚][3﹙x+y+z﹚-﹙x-y-z﹚]
=﹙3x+3y+3z+x-y-z﹚﹙3x+3y+3z-x+y+z﹚=﹙4x+2y+2z﹚﹙2x+4y+4z﹚
2a²x-2ax+½x=½x﹙4a²-4a+1﹚=½x﹙2a-1﹚²
¾﹙a+b﹚²+½﹙a+b﹚+¼=¼[﹙a+b﹚²+2﹙a+b﹚+1]=¼﹙a+b+1﹚²