Đáp án:
`7a)A=5x-x^2`
`=-(x^2-5x)`
`=-(x^2-5x+25/4-25/4)`
`=-[(x-5/2)^2-25/4]<=25/4`
Dấu "=" xảy ra khi `x=5/2`
`B=x-x^2`
`=-(x^2-x)`
`=-(x^2-x+1/4-1/4)`
`=-[(x-1/2)^2-1/4]<=1/4`
Dấu "=" xảy ra khi `x=1/2`
`C=4x-x^2+3`
`=-(x^2-4x)+3`
`=-(x^2-4x+4-4)+3`
`=-(x-2)^2+7<=7`
Dấu "=" xảy ra khi `x=2`
`D=-x^2+6x-11`
`=-(x^2-6x)-11`
`=-(x^2-6x+9-9)-11`
`=-(x-3)^2-2<=-2`
Dấu "=" xảy ra khi `x=3`
`E=5-8x-x^2`
`=-(x^2+8x)+5`
`=-(x^2+8x+16-16)+5`
`=-(x+4)^2+21<=21`
Dấu "=" xảy ra khi `x=-4`
`F=4x-x^2+1`
`=-(x^2-4x)+1`
`=-(x^2-4x+4-4)+1`
`=-(x-2)^2+5<=5`
Dấu "=" xảy ra khi `x=2`