Quadratic Function 11D1C1
1. **Problem Statement:** Given the function $f(x) = -x^2 - 3x + 28$, we need to find:
a) The $x$ and $y$ intercepts.
b) The coordinates of the vertex and its nature.
c) Plot the function.
d) Draw the axis of symmetry and find its equation.
2. **Finding the intercepts:**
- The $y$-intercept is found by evaluating $f(0)$:
$$f(0) = -(0)^2 - 3(0) + 28 = 28$$
So, the $y$-intercept is at $(0, 28)$.
- The $x$-intercepts are found by solving $f(x) = 0$:
$$-x^2 - 3x + 28 = 0$$
Multiply both sides by $-1$ to simplify:
$$x^2 + 3x - 28 = 0$$
Use the quadratic formula $x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$ where $a=1$, $b=3$, $c=-28$:
$$x = \frac{-3 \pm \sqrt{3^2 - 4(1)(-28)}}{2(1)} = \frac{-3 \pm \sqrt{9 + 112}}{2} = \frac{-3 \pm \sqrt{121}}{2}$$
$$x = \frac{-3 \pm 11}{2}$$
So,
$$x_1 = \frac{-3 + 11}{2} = 4$$
$$x_2 = \frac{-3 - 11}{2} = -7$$
The $x$-intercepts are at $(4, 0)$ and $(-7, 0)$.
3. **Finding the vertex:**
The vertex of a parabola $f(x) = ax^2 + bx + c$ is at $x = -\frac{b}{2a}$.
Here, $a = -1$, $b = -3$.
$$x_v = -\frac{-3}{2(-1)} = \frac{3}{-2} = -1.5$$
Find $y$ coordinate by evaluating $f(-1.5)$:
$$f(-1.5) = -(-1.5)^2 - 3(-1.5) + 28 = -2.25 + 4.5 + 28 = 30.25$$
So, the vertex is at $(-1.5, 30.25)$.
Since $a = -1 < 0$, the parabola opens downward, so the vertex is a maximum point.
4. **Axis of symmetry:**
The axis of symmetry is the vertical line passing through the vertex:
$$x = -1.5$$
5. **Plotting the function:**
The function is a downward opening parabola with vertex at $(-1.5, 30.25)$, $x$-intercepts at $(4,0)$ and $(-7,0)$, and $y$-intercept at $(0,28)$.
Final answers:
- $x$-intercepts: $(4, 0)$ and $(-7, 0)$
- $y$-intercept: $(0, 28)$
- Vertex: $(-1.5, 30.25)$ (maximum point)
- Axis of symmetry: $x = -1.5$