Question

Mn giúp em hai bài này với ạ

Answer 1

Bài `5:`

`P=2x^2-3x+4=2(x^2-3/2x+2)=2(x^2-2x. 3/4+9/16+23/16)=2(x-3/4)^2+23/8`

Vì `2(x-3/4)^2 >= 0 AA x<=>2(x-3/4)^2+23/8 >= 23/8 AA x`

   Hay `P >= 23/8 AA x`

Dấu "`=`" xảy ra `<=>(x-3/4)^2=0<=>x=3/4`

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Bài `6:`

`A=-3x^2+9x-2=-3(x^2+3x-2/3)=-3(x^2+2x. 3/2+9/4-35/12)=-3(x+3/2)^2+35/12`

Vì `-3(x+3/2)^2 <= 0 AA x<=>-3(x+3/2)^2+35/12 <= 35/12 AA x`

   Hay `A <= 35/12 AA x`

Dấu "`=`" xảy ra `<=>(x+3/2)^2=0<=>x=-3/2`