For positive a, b, c, that satisfy the condition ab+bc+ca=3, prove an inequality: 2a3+11+2b3+11+2c3+11≥1.
Solution
Let us make such transformation: 1=3ab+bc+ca≥3(abc)2⇔abc≤a≤bc1,b≤ac1,c≤ab1, hence a+b+c≤ab1+bc1+ca1. Then we use well-known inequality: b1a12+b2a22+⋯+bnan2≥b1+b2+⋯+bn(a1+a2+⋯+an)2. Then we will make such transformation: 2a3+11+2b3+11+2c3+11=2a+a21a21+2b+b21b21+2c+c21c21≥2a+2b+2c+a21+b21+c21(a1+b1+c1)2≥bc1+ca1+ab1+a21+b21+c212(a1+b1+c1)2=(a1+b1+c1)2(a1+b1+c1)2=1, That is what we had to prove.
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