Giải thích các bước giải:
a) `P=3sqrtx-x`
`=(-x+2sqrtx. 3/2-9/4)+9/4`
`=-(sqrtx-3/2)^2+9/4`
`=>P<=9/4`
Dấu = xảy ra `<=>sqrtx-3/2=0=>sqrtx=3/2=>x=9/4`
Vậy `Pmax=9/4<=>x=9/4.`
b) `Q=sqrt(9x^2+4-12x)+3x+5`
`=sqrt((3x)^2-2.3x.2+2^2)+3x+5`
`=sqrt((3x-2)^2)+3x+5`
`=|3x-2|+3x+5`
`=|2-3x|+3x+5`
Có `|2-3x|>=3x-2`
`=>|2-3x|+3x+5>=2-3x+3x+5=7`
`=>Q>=7`
Dấu = xảy ra `<=>2-3x>=0`
`=>3x<=2`
`=>x<=2/3`
Vậy `Qmin=7<=>x<=2/3.`
