Root transformation: If α and β are the roots of 4x^2 − 19x + 12 = 0, find the quadratic equation whose roots are 1/α and 1/β.

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:

  • Original equation: 4x^2 − 19x + 12 = 0 with roots α, β.
  • We want the equation with roots 1/α and 1/β.
  • Coefficients are real and the quadratic is non-degenerate.


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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