`đkxđ: 2018≤x≤2020`
`GTLN`
Áp dụng: `a+b≥2\sqrt(ab)` với `a=x-2018; b=2020-x`
`P=\sqrt(x-2018)+\sqrt(2020-x)`
`P=\sqrt[x-2018+2020-x+2\sqrt[(x-2018)(2020-x)]]`
`P≤\sqrt[2+(x-2018+2020-x)]`
`P≤\sqrt4=2`
Dấu `=` xảy ra `⇔x-2018=2020-x⇒x=2019`
Vậy `Max_P=2⇔x=2019`
$GTNN$
Áp dụng: `\sqrta+\sqrtb≥\sqrt(a+b)`
`P=\sqrt(x-2018)+\sqrt(2020-x)`
`P≥\sqrt(x-2018+2020-x)=\sqrt2`
Dấu `=` xảy ra `⇔x-2018=0` hoặc `2020-x=0`
`⇒x=2020; 2018`
Vậy $Min_P=\sqrt{2}⇔x=2018; 2020$
