2004 AMC 12B Problem 17

Attempt Problem 17 of the 2004 AMC 12B 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 2004 AMC 12B solutions, or check the answer key.

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17.

For some real numbers $a$ and $b,$ the equation $8x^3 + 4ax^2 + 2bx + a = 0$ has three distinct positive roots. If the sum of the base- $2$ logarithms of the roots is $5,$ what is the value of $a?$

$-256$

$-64$

$-8$

$64$

$256$

Answer: A

Concepts:Vieta’s Formulas logarithm

Difficulty rating: 1770

Solution:

The sum of the base- $2$ logarithms is $\log_2(r_1 r_2 r_3) = 5,$ so $r_1 r_2 r_3 = 2^5 = 32.$ By Vieta's formulas on $8x^3 + 4ax^2 + 2bx + a,$ the product of the roots is $-\dfrac{a}{8}.$ Thus $-\dfrac{a}{8} = 32,$ giving $a = -256.$

Thus, the correct answer is A.

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