How can you tell whether a rational expression is in simplest form? include an example with your explanation.

A rational in its most simple an be written as a fraction a/b where a and b are co-prime. Which means that there are no common factors in a and b 

for example 4/6  is not simplest form because  2 is a factor of 4 and 6
In its simplest form 4/6  is 2/3.

Two because two of one thing is two.