Question

How to find whether a quantity is the functions of r and t?

Answer 1

if any quantity is function of r than it is defined for r.for eg.
$f(r) = {r}^{2} + 1$
this function is defined for r.
if any function is defined for x than it depends on x only for eg
$f(x) = 2x + 3$
so that is defined in the function definition,that was defined for x and r