27a²-18a+3=3(9a²-6a+1)=3(3a-1)²x²+x+1/4 =(x+1/2)²9（a-b）²+24（a-b）+16=[3(a-b)+4]²=(3a-3b+4)²a的三次方-2a²+a=a(a²-2a+1)=a(a-1)²