Đáp án: + Giải thích các bước giải:
`a) (x+3)^2+x(5-x)=15`
`⇔ x^2 + 6x + 9 + 5x - x^2 = 15`
`⇔ 11x + 9 = 15`
`⇔ 11x = 6`
`⇔ x = 6/11`
Vậy `S = {6/11}`
`b) (x-1)(x+1)-(x+2)^2=11`
`⇔ x^2 - 1 - x^2 - 4x - 4 = 11`
`⇔ -4x - 5 = 11`
`⇔ -4x = 16`
`⇔ x = -4`
Vậy `S={-4}`
`c) (x-3)^2-(2x-5)^2=0`
`⇔ x^2 - 6x + 9 - 4x^2 + 20x - 25 = 0`
`⇔ -3x^2 + 14x - 16 = 0`
`⇔ -(3x^2-14x+16)=0`
`⇔ -(x-2)(3x-8)=0`
`⇔`\(\left[ \begin{array}{l}x-2=0\\3x-8=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=2\\x=\dfrac{8}{3}\end{array} \right.\)
Vậy `S = {2,8/3}`
