三道题目?
a的三次方-a=a(a²-1)=a(a²-1²)=a(a+1)(a-1)
3a方-3b方=3(a²-b²)=3(a+b)(a-b)
y方-9[x+y]的平方=y²-[3(x+y)]²=(y+3x+3y)(y-3x-3y)=(3x+4y)(-3x-2y)=-(3x+4y)(3x+2y)