20x^2-5y^2=5(4x^2-y^2),
4x^3-y^3=4(x^3-y^3),
4x^2y-4x^3-xy^2=x(4xy-4x^2-y^2);
232^2-231^2=(232+231)(232-231)=463,
148^2-2*148*48+48^2=(148-48)^2=100^2=10000,
(81^3-31^3)/(81^2+81*31+31^2)=(81-31)(81^2+81*31+31^2)/(81^2+81*31+31^2)=50;
48a^3-3ab^2=3a(16a^2-b^2)=3a(4a+b)(4a-b),
2x+y-4x^2-4xy-y^2=2x+y-(2x+y)^2=(2x+y)(1-2x-y),
(4-x)(4+x)-a(a+2x)=16-x^2-a^2+2ax=16-(x-a)^2=(4-x+a)(4+x-a);
x^3-3x^2-4x+12=0,
x^2(x-3)-4(x-3)=0,
(x-3)(x^2-4)=0,
(x-3)(x-2)(x+2)=0,
x1=3, x2=2, x3=-2