Find all real numbers for which the rational expression is undefined. (Enter your answers as a comma-separated list. If no values make the expression undefined, enter NONE.) 6x-19/x2 - 81

Answer: + 9, - 9

Step-by-step explanation: The rational expression in this case is undefined if the the denominator is equal to zero, which means that the x cannot be whatever makes the denominator zero, so:

x² - 81 = 0 ⇒ x² = 81 ⇒ x = + / - √81 ⇒ x = + / - 9

Note that we are using the equal sign just because this particular problem askes for a solutions that is undefined.