Find all numbers for which the rational expression is underfined:
r^2-4r?r^2-4r
If that means (r^2-4r)/(r^2-4r) then the value is undefined where
r^2-4r = 0
That is, when r(r-4) = 0
or, r = 0 or 4
r^2-4r?r^2-4r
r^2-4r = 0
That is, when r(r-4) = 0
or, r = 0 or 4