`1/(a^2+2b^2+3)+1/(b^2+2c^2+3)+1/(c^2+2a^2+3)`
`≤1/(2(ab+b+1))+1/(2(cb+c+1))+1/(2(ac+a+1))`
`=1/2 .(1/(ab+b+1)+1/(cb+c+1)+1/(ac+a+1)`
`=1/2 .(1/(ab+b+1)+(abc)/(abc+bc+c)+(acb)/(a^2bc+bac+ac))`
`=1/2.((ab+b+1)/(ab+b+1))`
`=1/2`
`"=" ` xẩy ra khi `:`
`a=b=c=1`