[tex]y=\dfrac{3}{x}\\\\\\
y'=\left(\dfrac{3}{x}\right)'=(3\cdot x^{-1})'=3\cdot(x^{-1})'=3\cdot(-1)\cdot x^{-2}=-3\cdot x^{-2}=\\\\\\=-\dfrac{3}{x^2}=-\dfrac{3}{x\cdot x}=-\dfrac{3}{x}\cdot\dfrac{1}{x}=-y\cdot\dfrac{1}{x}=-\dfrac{y}{x}[/tex]
Answer (A)
