y4次方-y的2次方=y^2*（y^2-1)=y^2(y+1)(y-1)
3ax²-3ay²=3a(x^2-y^2)=3a(x+y)(x-y)
4x三次方-8x²+4x=4x(x^2-2x+1)=4x(x-1)^2
a-2a(b+c)+(b+c)²=[a-(b+c)]^2=(a-b-c)^23q