AlgebraDifficulty 7.5National olympiad, round 2Prove it
35. Prove that for any positive real numbers a,b,c, we have 1<a2+b2a+b2+c2b+c2+a2c⩽232⋅(2004 China Western Mathematical Olympiad Problem)
Solution
35. The left side of the inequality is easy to prove: a2+b2a+b2+c2b+c2+a2c>a+b+ca+a+b+cb+a+b+cc=1
Now we prove the right side. By the Cauchy-Schwarz inequality, we have (a2+b2a+b2+c2b+c2+a2c)2=[a2+c2⋅(a2+b2)(a2+c2)a+b2+a2⋅(b2+c2)(b2+a2)b+c2+b2⋅(c2+a2)(c2+b2)c]2⩽[(a2+c2)+(b2+a2)+(c2+b2)]⋅[(a2+b2)(a2+c2)a2+(b2+c2)(b2+a2)b2+(c2+a2)(c2+b2)c2]=2(a2+b2+c2)[(a2+b2)(a2+c2)a2+(b2+c2)(b2+a2)b2+(c2+a2)(c2+b2)c2]
Now we prove (a2+b2)(a2+c2)a2(a2+b2+c2)+(b2+c2)(b2+a2)b2(a2+b2+c2)+(c2+a2)(c2+b2)c2(a2+b2+c2)⩽89(1)⇔8(a2+b2+c2)(a2b2+b2c2+c2a2)⩽9(a2+b2)(b2+c2)(c2+ a2)⇔a4b2+b4c2+c4a2+a2b4+b2c4+c2a4⩾6a2b2c2. This follows from the AM-GM inequality. Thus, 1<a2+b2a+b2+c2b+c2+a2c⩽232.
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