2006 AIME I Problem 5

Attempt Problem 5 of the 2006 AIME I 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 2006 AIME I solutions, or check the answer key.

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5.

The number 1046+46810+14415+2006\small \sqrt{104\sqrt{6} + 468\sqrt{10} + 144\sqrt{15} + 2006} can be written as a2+b3+c5,a\sqrt{2} + b\sqrt{3} + c\sqrt{5}, where a,a, b,b, and cc are positive integers. Find abc.a \cdot b \cdot c.

Answer: 936
Concepts:radicalsystem of equationsalgebraic manipulation
Difficulty rating: 2270
Solution:

Squaring a2+b3+c5a\sqrt{2} + b\sqrt{3} + c\sqrt{5} gives 2a2+3b2+5c2+2ab6+2ac10+2bc15. \begin{aligned} &2a^2 + 3b^2 + 5c^2 \\ &\quad {}+ 2ab\sqrt{6} + 2ac\sqrt{10} \\ &\quad {}+ 2bc\sqrt{15}. \end{aligned} Matching coefficients yields 2ab=104,2ab = 104, 2ac=468,2ac = 468, 2bc=144,2bc = 144, that is ab=52,ab = 52, ac=234,ac = 234, bc=72,bc = 72, along with 2a2+3b2+5c2=2006.2a^2 + 3b^2 + 5c^2 = 2006.

Then (abc)2=(ab)(ac)(bc)=5223472=876096=9362, \begin{aligned} (abc)^2 &= (ab)(ac)(bc) \\ &= 52 \cdot 234 \cdot 72 \\ &= 876096 = 936^2, \end{aligned} so abc=936.abc = 936. As a check, a=abcbc=13,a = \frac{abc}{bc} = 13, b=abcac=4,b = \frac{abc}{ac} = 4, c=abcab=18,c = \frac{abc}{ab} = 18, and 2169+316+5324=338+48+1620=2006, \begin{aligned} &2 \cdot 169 + 3 \cdot 16 + 5 \cdot 324 \\ &= 338 + 48 + 1620 \\ &= 2006, \end{aligned} as required.

Therefore abc=936.a \cdot b \cdot c = 936.

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