Tìm GTLN `A=15+7x-5x^2`

$\color{Orange}{\text{~Orange~}}$

`A=15+7x-5x^2`

`=-5(x^2-7/5x-3)`

`=-5(x^2-2*x*7/10+(7/10)^2-349/100)`

`=-5[(x-7/10)^2-349/100]`

`=-5(x-7/10)^2+349/20`

Vì `-5(x-7/10)^2≤0 forall x in RR`

`⇒-5(x-7/10)^2+349/20≤349/20`

Dấu "`=`" xảy ra khi

`x-7/10=0`

`⇒x=7/10`

Vậy `Max_A=349/20` khi `x=7/10`