(giả thiết `-a>=b-3`)
`P=b-a+20/a+7/b>=b-3+2/(3-b)+7/b=2b-3+20/(3-b)+7/b`
`=5(3-b)+20/(3-b)+7b+7/b-18>=2\sqrt{5.(3-b). 20/(3-b)}+2\sqrt{7b . 7/b}-18=16`
Dấu "`=`" xảy ra khi
`{(5(3-b)=20/(3-b)),(7b=7/b):}`
`=>b=1` thì `a=2`
Vậy `P_min=16` khi `b=1;a=2`