1996 AIME Problem 5

Attempt Problem 5 of the 1996 AIME 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 1996 AIME solutions, or check the answer key.

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5.

Suppose that the roots of x3+3x2+4x11=0x^3+3x^2+4x-11=0 are a,a, b,b, and c,c, and that the roots of x3+rx2+sx+t=0x^3+rx^2+sx+t=0 are a+b,a+b, b+c,b+c, and c+a.c+a. Find t.t.

Answer: 23
Concepts:Vieta’s Formulaspolynomialsymmetry (algebra)
Difficulty rating: 1710
Small Hint:

Use Vieta’s formulas on the original cubic

Big Hint:

Expand (a+b)(b+c)(c+a)(a+b)(b+c)(c+a) in symmetric sums

Solution:

Vieta’s formulas give a+b+c=3,ab+bc+ca=4,abc=11.\begin{aligned}a+b+c&=-3,\\ab+bc+ca&=4,\\abc&=11.\end{aligned} Also, (a+b)(b+c)(c+a)=(a+b+c)(ab+bc+ca)abc=23.\begin{gathered}(a+b)(b+c)(c+a)\\=(a+b+c)(ab+bc+ca)\\\quad-abc=-23.\end{gathered} This is the product of the roots of the second monic cubic, so its constant term is the negative of that product. Hence t=23.t=23.

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