28 tháng 9 lúc 23:04
\(x+\dfrac{5}{x}=7\)
\(\Rightarrow\left(x+\dfrac{5}{x}\right)^3=7^3\)
\(\Rightarrow x^3+\dfrac{125}{x^3}+3x.\dfrac{5}{x}\left(x+\dfrac{5}{x}\right)=7^3\)
\(\Rightarrow x^3+\dfrac{125}{x^3}+15.7=343\)
\(\Rightarrow x^3+\dfrac{125}{x^3}=238\)
\(\Rightarrow x^6+125=238x^3\)
\(\Rightarrow\dfrac{x^3}{x^6+125}=\dfrac{1}{238}\)
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