2021 AMC 10A Spring Problem 14

Below is the video solution and professionally curated solution for Problem 14 of the 2021 AMC 10A Spring, 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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Concepts:Vieta’s Formulaspolynomial

Difficulty rating: 1540

14.

All the roots of the polynomial z610z5+Az4+Bz3z^6-10z^5+Az^4+Bz^3+Cz2+Dz+16+Cz^2+Dz+16 are positive integers, possibly repeated. What is the value of B?B?

88-88

80-80

64-64

41-41

40-40

Video solution:
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Written solution:

By Vieta's formulas, we get that the product of the roots is 1616 and that their sum is 10.10.

Given that all the roots are positive integers, we can see that the roots are 1,1,2,2,2,2. 1, 1, 2, 2, 2, 2.

The polynomial is therefore (z1)2(z2)4=(z22z+1) (z - 1)^2(z - 2)^4 = (z^2 - 2z + 1) (z48z3+24z232z+16). (z^4 - 8z^3 + 24z^2 - 32z + 16).

Calculating just the z3z^3 term, we get 32z348z38z3=88z3. -32z^3 - 48z^3 - 8z^3 = -88z^3.

Thus, A is the correct answer.

Problem 14 in Other Years