Maths Olympiad Prep

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, 2013

Algebra Difficulty 8.4 Shortlist Prove it Saudi Arabia

Let TT be a real number satisfying the property: For any nonnegative real numbers a,b,c,d,ea, b, c, d, e with their sum equal to 11, it is possible to arrange them around a circle such that the products of any two neighboring numbers are no greater than TT. Determine the minimum value of TT.

Solution

Assume, without loss of generality, that
0abcde 0 \leq a \leq b \leq c \leq d \leq e
Because
eamax{eb,cd}eced e a \leq \max \{e b, c d\} \leq e c \leq e d
to get the smallest possible maximum, the best arrangement around the circle is
Figure 1
In this case, the maximum is given by
max{eb,cd}. \max \{e b, c d\} .
We have
ebeb+c+d3e(1e)3112, e b \leq e \frac{b+c+d}{3} \leq \frac{e(1-e)}{3} \leq \frac{1}{12},
and the equality holds when a=0a=0, b=c=d=16b=c=d=\frac{1}{6}, and e=12e=\frac{1}{2}.
We have, on the other hand,
cd=c2d2(cd)3(cde3)2(c+d+e3)219 c d=\sqrt[3]{c^{2} d^{2}(c d)} \leq (\sqrt[3]{c d e})^{2} \leq \left(\frac{c+d+e}{3}\right)^{2} \leq \frac{1}{9}
and the equality holds when a=b=0a=b=0, and c=d=e=13c=d=e=\frac{1}{3}.
Therefore, the minimum possible value of TT is 19\frac{1}{9}.

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