12a^2-4a-9a^3
=-a(9a^2-12a+4)
=-a(3a-2)^2
(x+y)^2-10(x+y)+25
=(x+y-5)^2
(x^2+y^2)^2-4x^2y^2.
=(x^2+y^2-2xy)(x^2+y^2+2xy)
=(x-y)^2(x+y)^2
(x+y)^2-2(x^2-y^2)+(x-y)^2
=(x+y-x+y)^2
=4y^2
十字相乘法x^2+六分之五+六分之一 (题目有误吧)
=x^2+1
x^5-10x^4+16x^3
=x^3(x^2-10x+16)
=x^3()(x-2)(x-8)
x^2y^2+20x^2y^2+64xy^2 题目有误
=21x^2y^2+64xy^2
=xy^2(21x+64)是加六分之五X十字相乘法x^2+六分之五+六分之一=(6x^2+5x+1)/6=(2x+1)(3x+1)/6