Sunday, 23 July 2017

inequality - Prove sumlimitscycfracsqrtxysqrtxy+zlefrac32 if x+y+z=1





if x,y,z are positive real numbers and x+y+z=1 Prove:cycxyxy+z32
where cyc denotes sums over cyclic permutations of the symbols x,y,z.



Additional info:I'm looking for solutions and hint that using Cauchy-Schwarz and AM-GM because I have background in them.




Things I have done so far: Using AM-GM xy+z2 xy+z2
So manipulating this leads to cycxyxy+zcycxy2




I stuck here.I'm thinking about applying Cauchy-Schwartz.Also I have not used the assumption x+y+z=1.Any hint is appreciated.


Answer



cycxyxy+z32
cycxyxy+z=cycxyxy+1xy=cycxy(1x)(1z)=cycxy(y+z)(x+z)
And now, by AM-GM
cycxy(y+z)(x+z)cycx2(x+z)+y2(y+z)=x2(x+z)+y2(y+z)+y2(y+x)+z2(z+x)+z2(z+x)+x2(x+y)=32


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