Đáp án:
`a+b+c=0`
`<=>(a+b+c)^2=0`
`<=>a^2+b^2+c^2+2ab+2bc+2ca=0`
`<=>a^2+b^2+c^2=-(2ab+2bc+2ca)`
`<=>(a^2+b^2+c^2)^2=(2ab+2bc+2ca)^2`
`<=>a^4+b^4+c^4+2a^2b^2+2b^2c^2+2c^2a^2=4a^2b^2+4b^2c^2+4c^2a^2+8ab^2c+8abc^2+8a^2bc`
`<=>a^4+b^4+c^4=2a^2b^2+2b^2c^2+2c^2a^2+8abc(a+b+c)`
`<=>a^4+b^4+c^4=2a^2b^2+2b^2c^2+2c^2a^2`
`<=>a^4+b^4+c^4+2a^2b^2+2b^2c^2+2c^2a^2=2a^4+2b^4+2c^4`
`<=>(a^2+b^2+c^2)^2=2(a^4+b^4+c^4)`