Đáp án + Giải thích các bước giải:
`3.` `d)` `(2x+3)^2-(x-1)^2=0`
`<=>[(2x+3)-(x-1)][(2x+3)+(x-1)]=0`
`<=>(2x+3-x+1)(2x+3+x-1)=0`
`<=>(x+4)(3x+2)=0`
`<=>`\(\left[ \begin{array}{l}x+4=0\\3x+2=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=-4\\x=-\dfrac{2}{3}\end{array} \right.\)
Vậy `S={-4;-2/3}`
Bài `4`
`a)` `(x+y)^2+(x-y)^2-2x^2`
`=x^2+2xy+y^2+x^2-2xy+y^2-2x^2`
`=2y^2`
`b)` `2(x-y)(x+y)+(x+y)^2+(x-y)^2`
`=(x-y)^2+2(x-y)(x+y)+(x+y)^2`
`=[(x-y)+(x+y)]^2`
`=(x-y+x+y)^2=(2x)^2=4x^2`
`c)` `(x-3)(x+3)-(x-5)^2`
`=x^2-9-(x^2-10x+25)`
`=x^2-9-x^2+10x-25=10x-34`.
