Phân thích thành nhân tử:
`x^2+20x-500`
`=x^2+25x-20x-500`
`=(x^2+25x)-(20x+500)`
`=x.(x+25)-20(x+25)`
`=(x-20).(x+25)`
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giải phương trình:
`x^2+5x-500=0`
`<=>x^2+25x-20x-500=0`
`<=>x.(x+25)-20.(x+25)=0`
`<=>(x-20).(x+25)=0`
`<=>`\(\left[ \begin{array}{l}x-20=0\\x+25=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=20\\x=-25\end{array} \right.\)
Vậy `S={20;-25}`
