a的3次方-4a的平方
=a(a^2-4a)
=a(a+/4a)(a-/4a)
4a^2+4ab+b^2-4a^2-4ab-a^2
=b^2-a^2
=(a+b)(a-b)
x^4+8x^2+16-16x^2
=x^4-8x^2+16
=(x2-4)^2
=(x+2)^2*(x-2)^2