Quadratic Inequalities 47B455
1. **State the problem:** Graph the quadratic inequalities:
a) $y > -x^2 - 3x - 4$
b) $-2y \geq x^2 - 1$
2. **Rewrite each inequality in graphing form (vertex form) by completing the square if needed.**
**a)** Start with $y > -x^2 - 3x - 4$
Rewrite the right side:
$$-x^2 - 3x - 4 = -(x^2 + 3x) - 4$$
Complete the square inside the parentheses:
Take half of 3, which is $\frac{3}{2}$, square it: $\left(\frac{3}{2}\right)^2 = \frac{9}{4}$
Add and subtract $\frac{9}{4}$ inside the parentheses:
$$-(x^2 + 3x + \frac{9}{4} - \frac{9}{4}) - 4 = -\left( (x + \frac{3}{2})^2 - \frac{9}{4} \right) - 4$$
Distribute the minus sign:
$$- (x + \frac{3}{2})^2 + \frac{9}{4} - 4 = - (x + \frac{3}{2})^2 - \frac{7}{4}$$
So the inequality becomes:
$$y > - (x + \frac{3}{2})^2 - \frac{7}{4}$$
This is the graphing form: a parabola opening downward with vertex at $\left(-\frac{3}{2}, -\frac{7}{4}\right)$.
3. **b)** Start with $-2y \geq x^2 - 1$
Divide both sides by $-2$, remembering to reverse the inequality sign because dividing by a negative number:
$$y \leq -\frac{1}{2}x^2 + \frac{1}{2}$$
This is already in graphing form with vertex at $(0, \frac{1}{2})$ and parabola opening downward.
4. **Graph shape description:**
- For (a), the parabola opens downward with vertex at $\left(-\frac{3}{2}, -\frac{7}{4}\right)$ and the region above the parabola is shaded (since $y$ is greater than the parabola).
- For (b), the parabola opens downward with vertex at $(0, \frac{1}{2})$ and the region below the parabola is shaded (since $y$ is less than or equal to the parabola).
5. **Summary:**
- (a) $y > - (x + \frac{3}{2})^2 - \frac{7}{4}$
- (b) $y \leq -\frac{1}{2}x^2 + \frac{1}{2}$
These forms are ready for graphing on a calculator or by hand.
Final answers:
$$\boxed{y > - (x + \frac{3}{2})^2 - \frac{7}{4}}$$
$$\boxed{y \leq -\frac{1}{2}x^2 + \frac{1}{2}}$$