`1, (x+4)^2-(x+1)(x-1)=16`
`⇔ x^2 + 8x + 16 - x^2 + 1 = 16`
`⇔ (x^2-x^2) + 8x = 16 - 16 - 1`
`⇔ 8x = -1`
`⇔ x = -1/8`
Vậy `S = {-1/8}`
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`2, (2x-1)^2 + (x+3)^2 - 5(x+7)(x-7) = 0`
`⇔ 4x^2 - 4x + 1 + x^2 + 6x + 9 - 5x^2 + 245 = 0`
`⇔ (5x^2-4x^2-x^2) + (6x-4x) = 0 - 245 - 9 - 1`
`⇔ 2x = -255`
`⇔ x = -255/2`
Vậy `S = {-255/2}`
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`3, 6x(x-4)+2x(2-3x)=-25`
`⇔ 6x^2 - 24x + 4x - 6x^2 = -25`
`⇔ (6x^2-6x^2) + (4x-24x) = -25`
`⇔ -20x = -25`
`⇔ x= 5/4`
Vậy `S = {5/4}`
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`4, 5x^2(3x-2)-3x^2(5x+2)+2x(3+8x)=21`
`⇔ 15x^3 - 10x^2 - 15x^3 - 6x^2 + 6x + 16x^2 = 21`
`⇔ (15x^3-15x^3)+(16x^2-6x^2-10x^2) + 6x = 21`
`⇔ 6x = 21`
`⇔ x = 7/2`
Vậy `S = {7/2}`
