Đáp án + Giải thích các bước giải:
`a)`
`(x+2)^{2}+(x-1)^{2}+(x-3)(x+3)-3x^{2}=-8`
`<=>x^{2}+4x+4+x^{2}-2x+1+x^{2}-9-3x^{2}=-8`
`<=>(x^{2}+x^{2}+x^{2}-3x^{2})+(4x-2x)+(4+1-9+8)=0`
`<=>2x+4=0`
`<=>2x=-4`
`<=>x=-2`
`b)`
`2021x(x-2020)-x+2020=0`
`<=>2021x(x-2020)-(x-2020)=0`
`<=>(x-2020)(2021x-1)=0`
`<=>` \(\left[ \begin{array}{l}x-2020=0\\2021x-1=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=2020\\x=\dfrac{1}{2021}\end{array} \right.\)
