Đáp án:
`A=9/(x^2-x+5)`
Vì `x^2-x+5`
`=x^2-x+1/4+19/4`
`=(x-1/2)^2+19/4>=19/4>0`
`=>A<=9/(19/4)=36/19`
Dấu "=" xảy ra khi `x-1/2=0<=>x=1/2`
Xét `B-4`
`=(27-12x-4^2-36)/(x^2+9)`
`=(-4x^2-12x-9)/(x^2+9)`
`=(-(2x+3)^2)/(x^2+9)<=0`
`=>B<=4`
Dấu "=" xảy ra khi `2x+3=0<=>x=-3/2`