AoPSWiki
Do you have what it takes to be the next brilliant trader, researcher, or developer at Jane Street Capital? Find out in the Careers in Mathematics Forum.

1983 AIME Problems/Problem 3

From AoPSWiki

Revision as of 01:14, 23 May 2009 by 5849206328x (Talk | contribs)
(diff) ← Older revision | Current revision (diff) | Newer revision → (diff)

Problem

What is the product of the real roots of the equation x^2 + 18x + 30 = 2 \sqrt{x^2 + 18x + 45}?

Solution

If we expand by squaring, we get a quartic polynomial, which obviously isn't very helpful.

Instead, we substitute y for x^2+18x+30 and our equation becomes y=2\sqrt{y+15}.

Now we can square; solving for y, we get y=10 or y=-6. The second solution is extraneous since 2\sqrt{y+15} is positive. So, we have y=10 as the only solution for y. Substituting x^2+18x+30 back in for y,

x^2+18x+30=10 \Longrightarrow x^2+18x+20=0.
By Vieta's formulas, the product of our roots is therefore \boxed{020}.

See also

1983 AIME (ProblemsResources)
Preceded by
Problem 2
Followed by
Problem 4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Looking for a challenging algebra text? Preparing for MATHCOUNTS or the AMC exams?
Check out Art of Problem Solving's Introduction to Algebra by Richard Rusczyk.
© Copyright 2008 AoPS Incorporated. All Rights Reserved. • FoundationPrivacyContact Us