Cho (x+y)^2 - 2(x^2+y^2) = 0

a,Cho (x+y)^2-2(x^2+y^2)=0 CM x=y
b,Cho(a+b+c)^2=3(ab+bc+ac) CMa=b=c
c,Cho a+b+c=0 CM 3abc-a^3-b^3-c^3=0