Is the rational expression defined at [[$ x = 1 $]]?

a) [[$ \displaystyle\frac {2x - 1} {3} $]]

Answer: The rational expression is is not defined at [[$ x = 1 $]]

b) [[$ \displaystyle\frac {5} {x - 1} $]]

Answer: The rational expression is is not defined at [[$ x = 1 $]]

c) [[$ \displaystyle\frac {x - 1} {2x} $]]

Answer: The rational expression is is not defined at [[$ x = 1 $]]

d) [[$ \displaystyle\frac {x} {x^2 + x - 2} $]]

Answer: The rational expression is is not defined at [[$ x = 1 $]]