To evaluate a rational expression, we substitute values of the variables into the expression and simplify, just as we have for many other expressions in this book. Explain how you find the values of x...To evaluate a rational expression, we substitute values of the variables into the expression and simplify, just as we have for many other expressions in this book. Explain how you find the values of x for which the rational expression \(\frac{x^2−x−20}{x^2−4}\) is undefined.