2 câu trả lời
`\sqrt[x-1]+\sqrt[9x-9]=20(ĐKXĐ:x>=1)`
`<=>\sqrt[x-1]+\sqrt[9(x-1)]=20`
`<=>\sqrt[x-1]+3\sqrt[x-1]=20`
`<=>4\sqrt[x-1]=20`
`<=>\sqrt[x-1]=5`
`<=>x-1=25`
`<=>x=26,tm`
Vậy `x=26`
$\sqrt{x-1}+\sqrt{9x-9}=20$
$⇔\sqrt{x-1}+3\sqrt{x-1}=20$
$ĐKXĐ:x-1≥0⇔x≥1$
$⇔\sqrt{x-1}(1+3)=20$
$⇔4\sqrt{x-1}=20$
$⇔\sqrt{x-1}=5$
$⇔x-1=25$
$⇔x=26(T/m)$
Vậy $S=${$26$}