Prove the inequality 4(3ba+3cb+3ac)≤3(2+a+b+c+a1+b1+c1)2/3 for positive real numbers a, b and c with abc=1.
Solutions — 2
Solution 1
Let x, y, z be positive real numbers such that a=x/y, b=y/z, c=z/x. After substitution and simplifying the given inequality is transformed to the following one: 4(x+y+z)3xyz≤3(x+y)2/3(y+z)2/3(z+x)2/3 For triangle with the sides lengths u=x+y, v=y+z, w=z+x the last inequality gives 4p3(p−u)(p−v)(p−w)≤3(uvw)2/3 where p is a semiperimeter of the triangle. By Heron's formula we obtain that the inequality is equivalent to: (2p)2/3≤(334suvw)2/3=(33R)2/3 where S is area and R is a radius of the circumcircle of the triangle. But the inequality u+v+w=2p≤33R is well-known; there are several ways to prove that.
Solution 2
By raising to the third power the given inequality and taking into account the condition abc=1 we eliminate radicals. After simplifying we get the following inequality: 114+30cyc∑(a+a1)+10cyc∑ba≤54cyc∑ab+27cyc∑(a2+a21) The last inequality can be obtained from the following elementary inequalities: 2cyc∑ba≤cyc∑(a2+a21),3≤cyc∑ab, a+a1≤41(a2+a21+6),2≤a2+a21.
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