Determine the point at which the graph of the function below has a horizontal tangent line. (If the function has no horizontal tangent line, enter NONE.)
f ( x ) = x³ + 3 x Slope of a horizontal line is 0. f ` ( x ) = 3 x² + 3 3 x² + 3 = 0 3 x² = - 3 / : 3 x² = - 1 , which give complex roots. Answer: NONE
