câu 19
`(2x-1)^2 -(5x-5)^2=0`
`<=> (2x-1-5x+5)(2x-1+5x-5)=0`
`<=> (4-3x)(7x - 6)=0`
`<=>`\(\left[ \begin{array}{l}4-3x=0\\7x-6=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\dfrac{4}{3}\\x=\dfrac{6}{7}\end{array} \right.\)
vậy`x ∈ {4/3; 6/7}`
`=>` chọn `C`
câu 20
`(2x+1)^2-4(x+3)^2=0`
`<=> (2x+1)^2 -(2x+6)^2=0`
`<=> (2x+1 - 2x-6)(2x+1+2x+6)=0`
`<=> -5(4x+7)=0 <=> 4x + 7=0`
`<=> 4x = -7`
`<=> x = -7/4 `
vậy` x = -7/4`
`=>` chọn `B`
câu 21
`(x-6)(x+6)-(x+3)^2=9`
`<=>x^2-36-x^2-6x-9-9=0`
`<=>-6x-54=0`
`<=>-6x=54`
`<=>x=-9`
vậy `x={-9}`
`=>` chọn `A`
câu 22
`(3x-1)^2+2(x+3)^2+11(1+x)(1-x)=6`
`<=>9x^2-6x+1+2(x^2+6x+9)+11(1-x^2)=6`
`<=>9x^2-6x+1+2x^2+12x+18+11-11x^2-6=0`
`<=>(9x^2+2x^2-11x^2)+(-6x+12x)+(1+18+11-6)=0`
`<=>6x+24=0`
`<=>6x=-24`
`<=>x=-4`
vậy `x={-4}`
`=>` chọn `B`
