3a+12a²+12a³=3a(1+4a+4a²)=3a(1+2a)²16-8（x-y)+(x-y)²=[4-(x-y)]²=(2-x+y)²a的4次幂b-6a³b+9a²b=a²b(a²-6a+9)=a²b(a-3)²