For positive numbers a, b, c, such that abc=1, prove the inequality: c2+a+ba2+b2+a2+b+cb2+c2+b2+c+ac2+a2≤2.
Solution
abc=1⇒31(a+b+c). Then: c2+a+b≤c2+(a+b)31(a+b+c)=31(3c2+a2+b2+2ab+ac+bc)≤61(8c2+5a2+5b2). So c2+a+ba2+b2+⋯≥61(8c2+5a2+5b2)a2+b2+⋯+8c2+5a2+5b26(a2+b2)+⋯≥2, the last inequality must be proved. As three points meant a cyclic permutation of variables. For simplification denote x=a2, y=b2, z=c2. Then we need to prove such inequality: A=8z+5x+5yx+y+⋯≥31. A=(x+y)(8z+5x+5y)(x+y)2+⋯≥(x+y)(8z+5x+5y)((x+y)2+…)2=10p+26q4p+8q≥31, this inequality supervenes from Cauchy-Schwarz inequality. There are used next denote: p=x2+y2+z2, q=xy+yz+zx. The last inequality equivalent p≥q, which are well known.
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