a^3+b^3+c^3-3abc=（a+b+c)(a²+b²+c²-ab-ac-bc)=3[(a+b+c)²-3（ab+ac+bc)]
=3(3²-3×6）=-27
a^4+b^4+c^4+d^4=4abcd
a^4-2a^2b^2+b^4 +c^4-2c^2d^2+d^4+2a^2b^2+2c^2d^2-4abcd=0
(a^2-b^2)^2+(c^2-d^2)^2+2(ab-cd)^2=0
平方为非负数和为0只有分别等于0
a=b,c=d ab=cd a=b=c=d 菱形或正方形
(3x-1)(x+1）=ax^2+2x-1 a=3