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Advanced Math / Nonlinear functions Difficulty: Hard

The function f is defined by f of x equals, open parenthesis, x plus 3, close parenthesis, times, open parenthesis, x plus 1, close parenthesis. The graph of f in the xy-plane is a parabola. Which of the following intervals contains the x-coordinate of the vertex of the graph of f ?

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Explanation

Choice B is correct. The graph of a quadratic function in the xy-plane is a parabola. The axis of symmetry of the parabola passes through the vertex of the parabola. Therefore, the vertex of the parabola and the midpoint of the segment between the two x-intercepts of the graph have the same x-coordinate. Since f of negative 3, equals, f of negative 1, which equals 0, the x-coordinate of the vertex is the fraction with numerator negative 3 plus negative 1, and denominator 2, equals negative 2. Of the shown intervals, only the interval in choice B contains –2. Choices A, C, and D are incorrect and may result from either calculation errors or misidentification of the graph’s x-intercepts.