Lưu
Câu `2:`
`a)`
`(x-5)^2-(x-3)(x+3)-2(3x+1)`
`=x^2-10x+25-(x^2-9)-6x-2`
`=x^2-10x+25-x^2+9-6x-2`
`=(x^2-x^2)-(10x+6x)+25+9-2`
`=32-16x`
`b)`
`(2x-3)^2+2(2x-3)(3x+2)+(3x+2)^2`
`=(2x-3+3x+2)^2`
`=(5x-1)^2`
`=25x^2-10x+1`
Câu `4:`
`a)`
`A=x^2+4x+13`
`=(x^2+4x+4)+9`
`=(x+2)^2+9`
Vì : `(x+2)^2 ge 0 \ AAx`
`to (x+2)^2+9 ge 9`
`to A ge 9`
Dấu "=" xảy ra khi : `(x+2)^2=0`
`<=> x=-2`
Vậy `A_(min)=9 <=> x=-2`
`b)`
`A=2x^2-6xy+9y^2-12x+2021`
`=(x^2-12x+36)+(x^2-6xy+9y^2)+1985`
`=(x-6)^2+(x-3y)^2+1985`
Vì : `(x-6)^2 ge 0 \ AAx ; (x-3y)^2 ge 0 \ AAx,y`
`to (x-6)^2+(x-3y)^2 ge0`
`to (x-6)^2+(x-3y)^2+1985 ge 1985>0 \ AAx,y`
