a. Let $k$ be a nonzero constant. If $y$ varies directly as $x,$ then $y=$ ____,where $k$ is the constant of variation.
b. Let $k$ be a nonzero constant. If $y$ varies inversely as $x,$ then $y=$ ____,where $k$ is the constant of variation.
c. Let $k$ be a nonzero constant. If $y$ varies jointly as $x$ and $w,$ then $y=$ ____, where $k$ is the constant of variation.