Maths Olympiad Prep

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Algebra Difficulty 5.2 AIME, harder Prove it JBMO

Problem:
Let x,y,zx, y, z be non-negative numbers such that x+2y+3z=1112x + 2y + 3z = \frac{11}{12}. Prove that
63xy+4x+2yz+6x+3y+4z+72xyz1718 63xy + 4x + 2yz + 6x + 3y + 4z + 72xyz \leq \frac{17}{18}
When does equality hold?

Solution

Solution:
Let x+2y+3z=1112x + 2y + 3z = \frac{11}{12}.

Using the notations a=bxa = bx, b=3b = 3, and c=2c = 2, from the equation we obtain the equality
2a+8b+9c=11. 2a + 8b + 9c = 11.

We also have
abc+ab+ac+bc+a+b+c+1=(a+1)(b+1)(c+1)×12518. a b c + a b + a c + b c + a + b + c + 1 = (a + 1)(b + 1)(c + 1) \times \frac{125}{18}.

By using the inequality
14(a+1+b1)=(2c+2)+6+8)0c+9=2a+2+b+8+9c+93)=(2a+8b+9c+13)=10 \begin{gathered} 14(a + 1 + b - 1) = (2c + 2) + 6 + 8) 0c + 9 = \\ \left.\frac{2a + 2 + b + 8 + 9c + 9}{3}\right) = \left(\frac{2a + 8b + 9c + 1}{3}\right) = 10 \end{gathered}

From the basic inequality, it follows that (a+1)(b+1)(c+1)12518(a + 1)(b + 1)(c + 1) \leq \frac{125}{18}.

Equality holds if 2a+8b+9c=102a + 8b + 9c = 10 or a=4,b=14a = 4, b = \frac{1}{4}, c=19c = \frac{1}{9}, that is x=23x = \frac{2}{3}, y=112y = \frac{1}{12}, z=130z = \frac{1}{30}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.