2014 AIME II Problems/Problem 5

Revision as of 18:03, 28 March 2014 by Suli (talk | contribs) (Created page with "==Solution== Let r, s, -r-s be the roots of p(x) (per Vieta's). Then <math>r^3 + ar + b = 0</math> and similarly for s. Also, <cmath>q(r+4) = (r+4)^3 + a(r+4) + b = 12r^2 + 48r +...")
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Solution

Let r, s, -r-s be the roots of p(x) (per Vieta's). Then $r^3 + ar + b = 0$ and similarly for s. Also, \[q(r+4) = (r+4)^3 + a(r+4) + b = 12r^2 + 48r + 64 + 4a = 0\] Set up a similar equation for s: \[q(s-3) = (s-3)^3 + a(s-3) + b + 240 = -9s^2 + 27s + 213 - 3a = 0\]. Simplifying and adding the equations gives \[3r^2 - 3s^2 + 12r + 9s + 87 = 0 (*)\] Now, let's deal with the a*x. Equating the a in both equations (per Vieta) \[rs + (-r-s)(r+s) = (r+4)(s-3) + (-r-s-1)(r+s+1)\], which eventually simplifies to \[s = \frac{13 + 5r}{2}\] Substitution into (*) should give r = -5 and r = 1, corresponding to s = -6 and s = 9, and |b| = 330 and 90, for an answer of $\boxed{420}$.