(x+y)²+6(x+y)+9=(x+y+3)²
a²－2a(b+c)+(b+c)²=[a-(b+c)]²=(a-b-c)²
4xy－4x²y－y³=-y(4x²-4xy+y²)=-y(2x-y)²
－a＋2a²－a³=-a(1-2a+a²)-a(1-a)²