`P=sum((1+a)^2(1+b)^2)/(1+c^2)`

`=sum((a+b)+(ab+1))^2/(1+c^2)`

`>=sum[4(a+b)(ab+1)]/(1+c^2)` (cauchy)

`=4sum(a^2b+a+ab^2+b)/(1+c^2)`

`=4sum(b(a^2+1)+a(b^2+1))/(1+c^2)`

`=4sum(b*(a^2+1)/(c^2+1)+a*(b^2+1)/(c^2+1))`

`=4sum(a*(b^2+1)/(c^2+1)+a*(c^2+1)/(b^2+1))`

`=4sum[a*((b^2+1)/(c^2+1)+(c^2+1)/(b^2+1))]`

`>=4sum(a*2)` (cauchy)

`=8suma`

`=8*3`

`=24`

Dấu "`=`" xảy ra khi: `a=b=c=1`