Quadratic Polynomial 55D781
1. **Problem statement:** A quadratic polynomial $p(x)$ has $(2x + 1)$ as a factor. When $p(x)$ is divided by $(x - 1)$ and $(x - 2)$, the remainders are $-6$ and $-5$ respectively. Find:
i) $p(x)$
ii) The zeros of $p(x)$
2. **Formula and rules:** Since $(2x + 1)$ is a factor, $p(x)$ can be written as $p(x) = (2x + 1)(ax + b)$ where $a$ and $b$ are constants to be determined.
3. **Using the remainder conditions:**
- When divided by $(x - 1)$, remainder is $p(1) = -6$
- When divided by $(x - 2)$, remainder is $p(2) = -5$
4. **Express $p(x)$:**
$$p(x) = (2x + 1)(ax + b) = 2a x^2 + (2b + a) x + b$$
5. **Evaluate $p(1)$ and $p(2)$:**
$$p(1) = 2a(1)^2 + (2b + a)(1) + b = 2a + 2b + a + b = 3a + 3b = -6$$
$$p(2) = 2a(2)^2 + (2b + a)(2) + b = 8a + 4b + 2a + b = 10a + 5b = -5$$
6. **Solve the system:**
From $3a + 3b = -6$, divide both sides by 3:
$$a + b = -2$$
From $10a + 5b = -5$, divide both sides by 5:
$$2a + b = -1$$
7. **Subtract equations:**
$$ (2a + b) - (a + b) = -1 - (-2) \\ a = 1$$
Substitute $a=1$ into $a + b = -2$:
$$1 + b = -2 \\ b = -3$$
8. **Write $p(x)$ explicitly:**
$$p(x) = (2x + 1)(1 imes x - 3) = (2x + 1)(x - 3)$$
Expanding:
$$p(x) = 2x^2 - 6x + x - 3 = 2x^2 - 5x - 3$$
9. **Find zeros of $p(x)$:** Solve $2x^2 - 5x - 3 = 0$ using quadratic formula:
$$x = \frac{5 \pm \sqrt{(-5)^2 - 4 \times 2 \times (-3)}}{2 \times 2} = \frac{5 \pm \sqrt{25 + 24}}{4} = \frac{5 \pm \sqrt{49}}{4}$$
$$x = \frac{5 \pm 7}{4}$$
So zeros are:
$$x = \frac{5 + 7}{4} = 3$$
$$x = \frac{5 - 7}{4} = -\frac{1}{2}$$
**Final answers:**
- i) $p(x) = 2x^2 - 5x - 3$
- ii) Zeros are $x = 3$ and $x = -\frac{1}{2}$