2021 AMC 10A Spring Problem 14

Attempt Problem 14 of the 2021 AMC 10A Spring below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Spring solutions, or check the answer key.

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14.

All the roots of the polynomial z610z5+Az4+Bz3+Cz2+Dz+16 \begin{aligned} &z^6-10z^5+Az^4+Bz^3\\ &\quad+Cz^2+Dz+16 \end{aligned} are positive integers, possibly repeated. What is the value of B?B?

88-88

80-80

64-64

41-41

40-40

Answer: A
Concepts:Vieta’s Formulaspolynomial
Difficulty rating: 1540
Video solution:
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Written solution:

By Vieta's formulas, the six roots have sum 1010 and product 16.16. Because the product is a power of 2,2, every positive integer root is a power of 2.2. Distributing the four factors of 22 among six roots gives the least possible sum when four roots are 22 and two roots are 11; that sum is already 10.10. Hence the roots are 1,1,2,2,2,2.1,1,2,2,2,2.

The coefficient BB is the negative of the sum of all products of three roots. Choosing zero, one, or two of the two roots equal to 11 gives B=((43)23+2(42)22+(41)2)=88. \begin{aligned} B&=-\left(\binom43 2^3+2\binom42 2^2\right.\\ &\qquad\left.+\binom41 2\right)\\ &=-88. \end{aligned}

Thus, A is the correct answer.

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