Đáp án: (Đổi đề: $168⇒169$)
$S=\{123\}$
Giải thích các bước giải:
`\frac{148-x}{25}+\frac{169-x}{23}+\frac{186-x}{21}+\frac{199-x}{19}=10`
`⇔(\frac{148-x}{25}-1)+(\frac{169-x}{23}-2)+(\frac{186-x}{21}-3)+(\frac{199-x}{19}-4)=0`
`⇔\frac{123-x}{25}+\frac{123-x}{23}+\frac{123-x}{21}+\frac{123-x}{19}=0`
`⇔(123-x)(\frac{1}{25}+\frac{1}{23}+\frac{1}{21}+\frac{1}{19})=0`
`⇔123-x=0` (do `\frac{1}{25}+\frac{1}{23}+\frac{1}{21}+\frac{1}{19}>0`)
`⇔x=123`