`E=5-2x-x^2`
`=-x^2-2x+5`
`=-x^2-2x-1+6`
`=-(x^2+2x+1)+6`
`=-(x+1)^2+6`
Với mọi `x\inRR.` ta có: `(x+1)^2>=0`
`<=>-(x+1)^2<=0`
`=>-(x+1)^2+6<=6`
Dấu `=` xảy ra khi `x+1=0<=>x=-1`
Vậy `GTLNN=6` khi `x=-1`
`F=5+6x-9x^2`
`=-9x^2+6x-1+6`
`=-(9x^2-6x+1)+6`
`=-(3x-1)^2+6`
Với mọi `x\inRR` ta có: `(3x-1)^2>=0`
`=>-(3x-1)^2<=0`
`<=>-(3x-1)^2+6<=6`
Dấu `=` xảy ra khi `3x-1=0<=>x=1/3`
Vậy `GTLNN=6` khi `x=1/3`
