1.a^3-7a-6
<=>x^3+2x^2-2x^2-4x-3x-6
<=>x^2-2x-3(x+2)=(x^2+x-3x-3)(x+2)
<=>[(x-3)(x+1)](x+2)
<=>(x-3)(x+1)(x+2)=0
<=>x-3=0 <=>x=3 hoặc x+1=0<=>x=-1 hoặc x+2=0<=>x=-2
2. a(b+c)^2+b(c+a)^2+c(a+b)^2-4abc
=a(b^2+2bc+c^2)+b(c^2+2ca+a^2)+c(a^2+2ab+b^2)-4abc
=ab^2+2abc+ac^2+bc^2+2abc+ba^2+ca^2+2abc+b^2-4abc
=ab^2+bc^2+ca^2+cb^2+6abc-4abc
=ab^2+bc^2+ca^2+cb^2+2abc
=a^3+b^3+c^3+2abc