`a)`
`A=-x^2-3x+10`
`=-(x^2+3x-10)`
`=-(x^2+2.x . 3/2+9/4-10-9/4)`
`=-[(x+3/2)^2-49/4]`
`=-(x+3/2)^2+49/4<=49/4`
Dấu "=" xảy ra khi : `(x+3/2)^2=0`
`<=> x+3/2=0`
`<=> x=-3/2`
Vậy `A_(max)=49/4 <=> x=-3/2`
`b)`
`B=-9x^2-6x-20`
`=-(9x^2+6x+20)`
`=-(9x^2+6x+1+19)`
`=-[(3x+1)^2+19]`
`=-(3x+1)^2-19<=-19`
Dấu "=" xảy ra khi : `(3x+1)^2=0`
`<=> x=-1/3`
Vậy `B_(max)=-19 <=> x=-1/3`
`c)`
`C=-9x(x+2/9)`
`=-9x^2-2x`
`=-(9x^2+2x)`
`=-(9x^2+2.3x. 1/3+1/9-1/9)`
`=-[(3x+1/3)^2-1/9]`
`=-(3x+1/3)^2+1/9<=1/9`
Dấu "=" xảy ra khi : `(3x+1/3)^2=0`
`<=> x=-1/9`
Vậy `C_(max)=1/9 <=> x=-1/9`
