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pructus
Posts:
2
From:
korea
Registered:
2/14/11


inequalities
Posted:
Feb 14, 2011 1:40 PM


Hi, Math geniuses!!
A question from an ordinary man.
The qestion is :
On given positive real numbers, find the range of real number k that satisfies ?(a+b)?^3 ?k?(a^3+b^3)?.
The answer of the book presents the answer is k?4.
But, when a=5, b=1 then k=2 does not satisfy the proposed anwer of k?4.
The logic of the book is for example p?q?r then p?r.
If this logic is valid, then if 2?x?4 then 2?4 is also valid?
I?d like to know if this logic is valid and the right answer of the proposed problem.



