Two differences: A polynomial inequality (PI) can always be replaced by a PI with one side zero. A rational inequality (RI) can not, because you do not know the sign of the denominator; a(x)b(x)>c(x)d(x) can not safely be replaced by a(x)d(x)−b(x)c(x)b(x)d(x)>0 because b(x) and/or d(x) might change signs unexpectedly.
