`3/(abc)>=suma^2`

`⇔3>=abc*suma^2`

`⇔3>=abc*[(suma)^2-2sumab]`

`⇔3>=abc*[9-2sumab]`

`⇔2abc*sumab+3>=9abc`

Áp dụng bất đẳng thức Cauchy:

`2abc*sumab+3`

`=abcsumab+abcsumab+3`

`>=3\root{3}{3*a^2b^2c^2*(sumab)^2}`

`>=3\root{3}{3*a^2b^2c^2*3abc*suma}`

`=3\root{3}{3*a^2b^2c^2*3abc*3}`

`=9abc`

Dấu "`=`" xảy ra khi: `a=b=c=1`