Quadratic Roots M
1. **State the problem:** We have the quadratic equation $x^2 - (m - 1)x + m + 2 = 0$ and want to find the values of $m$ for different root conditions.
2. **Recall discriminant formula:** The discriminant $\Delta$ determines the nature of roots.
$$\Delta = b^2 - 4ac$$
Here, $a=1$, $b=-(m-1)$, and $c=m+2$.
3. **Calculate the discriminant:**
$$\Delta = (-(m-1))^2 - 4(1)(m+2) = (m-1)^2 - 4(m+2)$$
Simplify:
$$= m^2 - 2m + 1 - 4m - 8 = m^2 - 6m - 7$$
4. **Case 1: No real roots** means:
$$\Delta < 0 \implies m^2 - 6m - 7 < 0$$
Solve the inequality by factoring the quadratic expression:
$$m^2 - 6m - 7 = (m - 7)(m + 1)$$
Check the inequality:
Between roots the expression is negative:
$$-1 < m < 7$$
5. **Case 2: Two distinct real roots** means:
$$\Delta > 0 \implies m^2 - 6m - 7 > 0$$
This holds outside the roots of the parabola:
$$m < -1 \text{ or } m > 7$$
6. **Case 3: Two distinct roots of opposite signs** means:
- The product of roots must be negative.
- From quadratic formula:
Product of roots = $c/a = m+2$
So:
$$m + 2 < 0 \implies m < -2$$
Also from case 2, roots must exist (discriminant $>0$).
Check which $m$ satisfy both:
$$m < -2 \\ \text{and} \\ (m < -1 \text{ or } m > 7)$$
So it is:
$$m < -2$$
7. **Case 4: Two strictly positive distinct roots** means:
- Roots are distinct: $\Delta > 0$
- Both roots positive: sum and product of roots are positive.
Sum of roots: $\frac{-b}{a} = m-1$
Product of roots: $\frac{c}{a} = m + 2$
Conditions:
$$m - 1 > 0 \implies m > 1$$
$$m + 2 > 0 \implies m > -2$$
Together with $\Delta > 0 \implies m < -1 \text{ or } m > 7$
Combine all:
$$m > 7$$
8. **Case 5: Two strictly negative distinct roots** means:
- Roots are distinct: $\Delta > 0$
- Both roots negative: sum and product of roots are positive and negative respectively:
Sum of roots $= m - 1 < 0 \implies m < 1$
Product of roots $= m + 2 > 0 \implies m > -2$
With $\Delta > 0$, which is $m < -1$ or $m > 7$.
Combine all:
$$-2 < m < -1$$
9. **Case 6: Double root** means:
$$\Delta = 0 \implies m^2 - 6m - 7 = 0$$
Solve:
$$m = \frac{6 \pm \sqrt{36 + 28}}{2} = \frac{6 \pm \sqrt{64}}{2} = \frac{6 \pm 8}{2}$$
Possible values:
$$m = 7 \quad \text{or} \quad m = -1$$
**Summary:**
1) No real roots: $-1 < m < 7$
2) Two distinct real roots: $m < -1$ or $m > 7$
3) Two distinct roots of opposite signs: $m < -2$
4) Two strictly positive distinct roots: $m > 7$
5) Two strictly negative distinct roots: $-2 < m < -1$
6) Double root: $m = -1$ or $m = 7$