1963 AMC 12 Problem 28

Attempt Problem 28 of the 1963 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1963 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

28.

Given the equation 3x2−4x+k=03x^2-4x+k=0 with real roots. The value of kk for which the product of the roots of the equation is a maximum is:
169\dfrac{16}{9}
163\dfrac{16}{3}
49\dfrac49
43\dfrac43
−43-\dfrac43
Answer: D
Concepts:quadraticVieta’s Formulasoptimization
Small hint:
The product of the roots is k3\frac{k}{3}
Big hint:
Use the discriminant condition to find the largest allowed kk
Solution:
Real roots require 16−12k≥0,16-12k\geq0, so k≤43.k\leq\frac{4}{3}. The root product is k3,\frac{k}{3}, which increases with k.k. It is therefore largest at k=43.k=\frac{4}{3}.
Thus, the correct answer is D.
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