Đáp án: $f\left(\dfrac13\right)=\dfrac{71}{15}$
Giải thích các bước giải:
Với $x=3$
$\to 2f\left(3\right)-f\left(\dfrac13\right)=2\cdot 3^2-1$
$\to 2f\left(3\right)-f\left(\dfrac13\right)=17$
$\to f\left(\dfrac13\right)=2f\left(3\right)-17$
Với $x=\dfrac13$
$\to 2f\left(\dfrac13\right)-f\left(\dfrac1{\dfrac13}\right)=2\cdot \left(\dfrac13\right)^2-1$
$\to 2f\left(\dfrac13\right)-f\left(3\right)=-\dfrac79$
$\to 2\cdot \left(2f\left(3\right)-17\right)-f\left(3\right)=-\dfrac79$
$\to 4f\left(3\right)-34-f\left(3\right)=-\dfrac79$
$\to 3f\left(3\right)=\dfrac{163}5$
$\to f\left(3\right)=\dfrac{163}{15}$
$\to f\left(\dfrac13\right)=2\cdot \dfrac{163}{15}-17=\dfrac{71}{15}$
