Difficulty: Medium
Correct Answer: 12x^2 - 19x + 4 = 0
Explanation:
Introduction / Context: This tests the reciprocal-root transformation. Given a quadratic with roots α and β, we are to construct the equation for roots 1/α and 1/β without explicitly finding α and β.
Given Data / Assumptions:
Concept / Approach: For ax^2 + bx + c = 0, sum α + β = −b/a and product αβ = c/a. Then for reciprocals: (1/α) + (1/β) = (α + β)/(αβ) and (1/α)(1/β) = 1/(αβ). The monic form is x^2 − [(α + β)/(αβ)]x + [1/(αβ)] = 0, which can be scaled to clear denominators.
Step-by-Step Solution:
a = 4, b = −19, c = 12.α + β = −b/a = 19/4; αβ = c/a = 12/4 = 3.For reciprocals: sum = (α + β)/(αβ) = (19/4)/3 = 19/12; product = 1/(αβ) = 1/3.Equation: x^2 − (19/12)x + 1/3 = 0 → multiply by 12: 12x^2 − 19x + 4 = 0.Verification / Alternative check: Direct transformation rule: cx^2 + bx + a = 0 for reciprocals gives 12x^2 − 19x + 4 = 0, consistent.
Why Other Options Are Wrong: Signs or placements of coefficients are incorrect; only option with pattern c, b, a works.
Common Pitfalls: Forgetting to divide by product αβ when summing reciprocals; not clearing denominators cleanly.
Final Answer: 12x^2 − 19x + 4 = 0
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