12ab-6(a^2+b^2)=-6(a^2-2ab+b^2)=-6(a-b)^2
3x^2+5xy-2y^2+x+9y-4=(3x-y)(x+2y)+x+9y-4=(3x-y+4)(x+2y-1)a3+1怎么分a^3+1=(a+1)(a^2-a+1)