Prove that for all positive x, y, and z,
(x+y)^z+(y+z)^x+(z+x)^y > 2.
Without loss of generalization, we can assume that x > y > z.
So, (x + y) > 2y > 2z.
And (x + z) > 2z.
And (y + z) > 2z.
Therefore the sum at left (let's call it S) is greater than :
(2z)^z + (2z)^z + (2z)^z = 3 x (2z)^z = 3 x 2^z x z^z
S > 3 x 2^z x z^z
When z tends to zero :
lim (S) = 3 x lim (2^z) x lim z^z, each limit when z-->0.
Since lim (2^z) = 1, and lim (z^z) is also = 1, then :
lim (S) = 3, when z--> 0.
Therefore, S > 2, as asked, and indeed >= 3.
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Posted by pcbouhid
on 2005-03-15 22:23:16 |