áp dụng co-si
`\Sigma a/(b+c)=\Sigma a^2/(ba+ca) ≥(a+b+c+d)^2/(ab+da+bc+cd+2ac+2db)`
`⇔\Sigma a/(b+c)≥((a+c)^2+(b+d)^2+2(a+c)(b+c))/(ab+da+bc+cd+2ac+2db)`
`⇔\Sigma a/(b+c)≥(4ac+4bd+2(ab+da+bc+cd))/(ab+da+bc+cd+2ac+2db)`
`⇔\Sigma a/(b+c)≥(2(ab+da+bc+cd+2ac+2db))/(ab+da+bc+cd+2ac+2db)`
`⇔\Sigma a/(b+c)≥2`
`''=''` khi :
`a=b=c=d`