ĐK:\(x\ge-1\)
\(PT\Leftrightarrow5\sqrt{x+1}=15\Leftrightarrow\sqrt{x+1}=3\Leftrightarrow x+1=9\Leftrightarrow x=8\)
\(\sqrt{25\left(x+1\right)}=15\)
\(\Leftrightarrow25\left(x+1\right)=225\)
\(\Leftrightarrow x+1=9\)
\(\Leftrightarrow x=8\)
Vậy chọn đáp án A.