AlgebraDifficulty 7.1National olympiad, round 2Prove it
□ Example 4 Given 5n real numbers ri,si,ti,ui,vi all greater than 1(1⩽i⩽n), let R=(n1∑i=1nri),S=(n1∑i=1nsi),T=(n1∑i=1nti),U=(n1∑i=1nui),V=(n1∑i=1nvi), prove that: ∏i=1n(risitiuivi−1risitiuivi+1)⩾(RSTUV−1RSTUV+1)n. (1994 Chinese National Training Team Problem)
Solution
Proof: Let x1,x2,⋯,xn∈(1,+∞), then by the generalized Cauchy inequality we have (1+x1)(1+x2)⋯(1+xn)⩾(1+nx1x2⋯xn)n
i.e., n(1+x1)(1+x2)⋯(1+xn)⩾1+nx1x2⋯xn. In (1), replace xi with xi−1 to get n(x1−1)(x2−1)⋯(xn−1)⩽nx1x2⋯xn−1 (1) ÷ (2) gives n(x1−1)(x2−1)⋯(xn−1)(x1+1)(x2+1)⋯(xn+1)⩾nx1x2⋯xn−1nx1x2⋯xn+1
Take xi=risitiuivi(i=1,2,⋯,n) to get i=1∏nrisitiuivi−1risitiuivi+1⩾(∏i=1nrisitiuivi−1∏i=1nrisitiuivi+1)
By the AM-GM inequality, we have ni=1∏nri⩽n1i=1∑nri=R
Similarly, n∏i=1nsi⩽S,n∏i=1nti⩽T,n∏i=1nui⩽U,n∏i=1nvi⩽V, so ni=1∏nrisitiuivi⩽RSTUV
Since y=x−1x+1 is a decreasing function on (1,+∞), we have ∏i=1nrisitiuivi−1∏i=1nrisitiuivi+1⩾RSTUV−1RSTUV+1
By (3) and (4), the original inequality holds.
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