Solution
Related Formula
For both roots of a quadratic equation ax² + bx + c = 0 to be negative real numbers, three mandatory rules must be met simultaneously:
- D ≥ 0 (Real roots)
- Sum of roots = -b/a < 0
- Product of roots = c/a > 0
Core Logic
From the given quadratic equation x² - (p + 2)x + (2p + 9) = 0:
Condition 1: Discriminant D ≥ 0
D = [-(p + 2)]² - 4(1)(2p + 9) ≥ 0 p² + 4p + 4 - 8p - 36 ≥ 0 p² - 4p - 32 ≥ 0 (p - 8)(p + 4) ≥ 0 p in (-∞, -4] [8, ∞) (i)Step 1: Evaluate Sum and Product Conditions
Condition 3: Product of roots > 0
αβ = 2p + 9 > 0 p > -(9)/(2) (iii)Step 2: Find Intersection Domain
Take the operational intersection across all three parameters: (i), (ii), and (iii):
- From (ii) and (iii): p in (-(9)/(2), -2)
- Intersecting this with (i) limits the range cleanly to:
Thus, α = -(9)/(2) and β = -4.
Step 3: Final Value Calculation
Calculate the requested target expression:
β - 2α = -4 - 2(-(9)/(2)) = -4 + 9 = 5Pattern Recognition
Remember that if roots are strictly real and matching signs, managing product rules before analyzing spatial configurations saves major compute overhead during intersection evaluation.
Chapter Mix
Class 11 Mathematics: Complex Numbers and Quadratic Equations