65. (48th International Mathematical Olympiad China National Training Team Test Question, March 2007) Let positive real numbers satisfy . Prove that
Solution
65. Proof: Let , then the original proposition is equivalent to: , and , then
The proof of equation (1) can be found in Exercise 64 (1) of this chapter.
Attachment: The solution provided by the National Training Team for this problem (provided by the problem group).
Proof: Let be positive real numbers, then the condition becomes
The conclusion becomes
Since must have two numbers on the same side of 1, without loss of generality, assume are on the same side of 1, then
Thus,
From this, we get
Therefore,
Notice that we need to prove
Next, we prove Equation (※※). By Equation (1), the left side ; by Equation (2),
the right side hence Equation (※※) holds.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.