a: (a+b+c)^2+a^2+b^2+c^2
=a^2+b^2+c^2+a^2+b^2+c^2+2ab+2ac+2bc
=(a^2+2ab+b^2)+(b^2+2bc+c^2)+(a^2+2ac+c^2)
=(a+b)^2+(b+c)^2+(c+a)^2
b: (x+y)^4-2(x^2+xy+y^2)^2
=(x^2+2xy+y^2)^2-2(x^2+xy+y^2)^2
=x^4+4x^2y^2+y^4+4x^3y+2x^2y^2+4xy^3-2(x^4+x^2y^2+y^4+2x^3y+2x^2y^2+2xy^3)
=-x^4-y^4
=>ĐPCM