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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

59. Let a1,a2,,ana_{1}, a_{2}, \cdots, a_{n} be positive real numbers, and a1+a2++an=1a_{1}+a_{2}+\cdots+a_{n}=1, denote Hk=H_{k}= k1a1+1a2++1ak(k=1,2,,n)\frac{k}{\frac{1}{a_{1}}+\frac{1}{a_{2}}+\cdots+\frac{1}{a_{k}}}(k=1,2, \cdots, n), prove: H1+H2++Hn<2H_{1}+H_{2}+\cdots+H_{n}<2. (2004 Polish Mathematical Olympiad Problem)

Solution

59. First prove
Hk4k(k+1)2(a1+4a2++k2ak)H_{k} \leqslant \frac{4}{k(k+1)^{2}}\left(a_{1}+4 a_{2}+\cdots+k^{2} a_{k}\right)

By the Cauchy-Schwarz inequality, we have
(1a1+1a2++1ak)(a1+4a2++k2ak)(1+2++k)2=k2(k+1)24\begin{array}{l} \left(\frac{1}{a_{1}}+\frac{1}{a_{2}}+\cdots+\frac{1}{a_{k}}\right)\left(a_{1}+4 a_{2}+\cdots+k^{2} a_{k}\right) \geqslant \\ (1+2+\cdots+k)^{2}=\frac{k^{2}(k+1)^{2}}{4} \end{array}

Therefore,
Hk4k(k+1)2(a1+4a2++k2ak)H1+H2++Hnα1a1+α2a2++αnan\begin{array}{c} H_{k} \leqslant \frac{4}{k(k+1)^{2}}\left(a_{1}+4 a_{2}+\cdots+k^{2} a_{k}\right) \\ H_{1}+H_{2}+\cdots+H_{n} \leqslant \alpha_{1} a_{1}+\alpha_{2} a_{2}+\cdots+\alpha_{n} a_{n} \end{array}

Thus,
where
αi=4i2(1i(i+1)2+1(i+1)(i+2)2++1n(n+1)2),i=1,2,,n\alpha_{i}=4 i^{2}\left(\frac{1}{i(i+1)^{2}}+\frac{1}{(i+1)(i+2)^{2}}+\cdots+\frac{1}{n(n+1)^{2}}\right), i=1,2, \cdots, n

Also,
1m(m+1)2<122m+1m2(m+1)2=12(1m21(m+1)2)\frac{1}{m(m+1)^{2}}<\frac{1}{2} \cdot \frac{2 m+1}{m^{2}(m+1)^{2}}=\frac{1}{2}\left(\frac{1}{m^{2}}-\frac{1}{(m+1)^{2}}\right)

Therefore,
αi<4i212[(1i21(i+1)2)+(1(i+1)21(i+2)2)++(1n21(n+1)2)]<4i212(1i21(n+1)2)<4i212i2=2\begin{array}{l} \alpha_{i}<4 i^{2} \cdot \frac{1}{2}\left[\left(\frac{1}{i^{2}}-\frac{1}{(i+1)^{2}}\right)+\left(\frac{1}{(i+1)^{2}}-\right.\right. \\ \left.\left.\frac{1}{(i+2)^{2}}\right)+\cdots+\left(\frac{1}{n^{2}}-\frac{1}{(n+1)^{2}}\right)\right]< \\ 4 i^{2} \cdot \frac{1}{2}\left(\frac{1}{i^{2}}-\frac{1}{(n+1)^{2}}\right)<4 i^{2} \cdot \frac{1}{2 i^{2}}=2 \end{array}

Thus,
H1+H2++Hnα1a1+α2a2++αnan<2(a1+a2++an)=2\begin{array}{l} H_{1}+H_{2}+\cdots+H_{n} \leqslant \alpha_{1} a_{1}+\alpha_{2} a_{2}+\cdots+\alpha_{n} a_{n}< \\ 2\left(a_{1}+a_{2}+\cdots+a_{n}\right)=2 \end{array}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.