Quadratic with misread coefficients: For the true equation x^2 + p x + q = 0, one solver (wrong p) found roots 2 and 6; another solver (wrong q) found roots 2 and −9. Determine the correct roots and the correct quadratic.

Difficulty: Medium

Correct Answer: -3, -4 and x^2 + 7x + 12

Explanation:


Introduction / Context:
The question leverages Vieta’s formulas: for a monic quadratic x^2 + px + q = 0, the sum of roots is −p and the product is q. Using the information about the two incorrect attempts, we can deduce the true p and q, and hence the correct roots and equation.


Given Data / Assumptions:

  • True equation: x^2 + p x + q = 0.
  • Wrong-p attempt yielded roots 2 and 6, so that attempt used the same q but incorrect p.
  • Wrong-q attempt yielded roots 2 and −9, so that attempt used the same p but incorrect q.


Concept / Approach:
From the wrong-p attempt: product of roots is still q, thus q = 2*6 = 12. From the wrong-q attempt: sum of roots is still −p, thus −p = 2 + (−9) = −7 ⇒ p = 7. With p and q known, solve the correct quadratic for actual roots.


Step-by-Step Solution:

Wrong-p product ⇒ q = 12.Wrong-q sum ⇒ −p = −7 ⇒ p = 7.Correct equation: x^2 + 7x + 12 = 0.Factor: (x + 3)(x + 4) = 0 ⇒ roots are −3 and −4.


Verification / Alternative check:

Check Vieta: sum −3 + (−4) = −7 = −p; product (−3)(−4) = 12 = q. Consistent.


Why Other Options Are Wrong:

  • Other choices list roots/equations that do not match p = 7 and q = 12 determined from the two attempts.


Common Pitfalls:

  • Mixing up which parameter (p or q) remains correct in each incorrect attempt.
  • Forgetting signs in the sum −p and product q relations.


Final Answer:

-3, -4 and x^2 + 7x + 12

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