## Thomas' Calculus 13th Edition

Equate $f(x,y)=c$ for each value of c $xy=c\quad$or For $c=0,$ we have the origin. For the other values of c, $y=\displaystyle \frac{c}{x}$ is a family of hyperbolas related to $y=\displaystyle \frac{1}{x}$, by performing reflections about the x-axis and vertical stretching.