1.a的三次方-9a
=a(a²-9)
=a(a+3)(a-3)
2.x(x-1)-3x+4
=x²-4x+4
=(x-2)²
3.x的平方+2x+1
=(x+1)²
3.ax的平方-ay的平方
=a(x²-y²)
=a(x+y)(x-y)
4.a的平方-3a
=a(a-3)