where a, b, and c are real numbers and a ≠ 0 . The quadratic term is ax 2 , the linear term is bx, and the constant term is c. Quadratic functions have degree two. The graph of a quadratic function is called a parabola. If a < 0, then the parabola opens downward, like function g. If a > 0, then the parabola opens upward like function f. If the parabola opens upward, then the vertex is the point whose yvalue is the minimum value of f. If the parabola opens downward, then the vertex is the point whose y-value is the maximum value of f. The vertical line that goes through the vertex is called the axis of symmetry of the parabola.