phương trình hệ x hay phương trinhg hệ f vậy?
\(4\left(x^3+\frac{1}{x^3}\right)=13\left(x+\frac{1}{x}\right)\)
\(\Leftrightarrow4.\frac{x^6+1}{x^3}=13.\frac{x^2+1}{x}\)
\(\Leftrightarrow\frac{4x^6+4}{x^3}=\frac{13x^2+13}{x}\)
\(\Leftrightarrow\frac{4x^6+4}{x^3}=\frac{13x^4+13x^2}{x^3}\)
\(\Leftrightarrow4x^6+4=13x^4+13x^2\)
............
\(x+\frac{1}{x}=a\)
\(x^3+\frac{1}{x^3}=\left(x+\frac{1}{x}\right)\left[\left(x+\frac{1}{x}\right)^2-3\right]=a\left(a^2-3\right)\)