정답
The vertex is at $(h, k)$ where $h = -\frac{b}{2a}$ and $k = f\left(-\frac{b}{2a}\right)$. The axis of symmetry is the vertical line $x = -\frac{b}{2a}$.
해설
If $a > 0$, the parabola opens upward and $k$ is the minimum value; if $a < 0$, the parabola opens downward and $k$ is the maximum value.