What are rational expressions? Why must we always be mindful of the final value of the denominator in a ration expression?

What are rational expressions? Why must we always be mindful of the final value of the denominator in a ration expression? For example, consider the rational expression 3x(x^2-16). What values in the denominator must be we be mindful of? Explain why. (minimum 200 words)

Rational expression: Any expression of the form p/q where p and q are integers with q not equal to 0, is called a rational expression. If q=0 the fraction p/q goes to infinity and we can't work...