Đáp án+Giải thích các bước giải:
`1)A=2021-4x^2+6x`
`<=>-A=4x^2-6x-2021`
`<=>-A=(2x)^2-2*2x*3/2+9/4-9/4-2021`
`<=>-A=(2x-3/2)^2-8093/4>=-8093/4`
`=>A<=8093/4`
Dấu "=" xảy ra khi `2x=3/2<=>x=3/4.`
`2)B=9x^2+4y^2+6y+2x-5`
`<=>B=(3x)^2+2*3x*1/3+1/9+4y^2+2*2y*3/2+9/4-1/9-9/4-5`
`<=>B=(3x+1/3)^2+(2y+3/2)^2-265/36>=-265/36`
Dấu "=" xảy ra khi `3x+1/3=0,2y+3/2=0<=>x=-1/9,y=-3/4.`