`A=8x-x^2`
`⇔A=-(x^2-8x+16)+16`
`⇔A=-(x-4)^2+16≤16`
`''=''`xẩy ra khi :
`x-4=0`
`⇔x=4`
vậy `maxA=16` khi `x=4`
`B=-x^2+10x-23`
`⇔B=-(x^2-10x+25)+2`
`⇔B=-(x-5)^2+2≤2`
`''=''`xẩy ra khi :
`x-5=0`
`⇔x=5`
vậy `maxA=2` khi `x=5`
`C=(24)/(x-2.\sqrt(x-4)+11)`
`⇔C=(24)/(((x-4)-2.\sqrt(x-4)+1)+14)`
`⇔C=(24)/((\sqrt(x-4)-1)^2+14)≤(24)/(0+14)≤(24)/(14)`
`''=''`xẩy ra khi :
`\sqrt(x-4)-1=0`
`⇔\sqrt(x-4)=1`
`⇔x=5`
vậy ........
`D=(32)/(x-4.\sqrt(x-3)+10)`
`⇔D=(32)/(((x-3)-2.\sqrt(x-3).2+4)+9)`
`⇔D=(32)/((\sqrt(x-3)-2)^2+9)≤(32)/(+9)≤(32)/(9)`
`''=''`xẩy ra khi :
`x-3=2^2=4`
`⇔x=7`
vậy ........
