`(a+b+c)^2>=3(ab+bc+ca)`
`<=>a^2+b^2+c^2+2ab+2bc+2ca>=3ab+3bc+3ca`
`<=>a^2+b^2+c^2>=ab+bc+ca`
`<=>2a^2+2b^2+2c^2>=2ab+2bc+2ca`
`<=>a^2-2ab+b^2+b^2-2bc+c^2+c^2-2ca+a^2>=0`
`<=>(a-b)^2+(b-c)^2+(c-a)^2>=0` luôn đúng
Dấu "=" xảy ra khi `a=b=c`
`a^2+b^2+c^2>=(a+b+c)^2/3`
`<=>3(a^2+b^2+c^2)>=(a+b+c)^2`
`<=>3a^2+3b^2+3c^2>=a^2+b^2+c^2+2ab+2bc+2ca`
`<=>2a^2+2b^2+2c^2>=2ab+2bc+2ca`
`<=>a^2-2ab+b^2+b^2-2bc+c^2+c^2-2ca+a^2>=0`
`<=>(a-b)^2+(b-c)^2+(c-a)^2>=0` luôn đúng
Dấu "=" xảy ra khi `a=b=c`
