Let , and be real numbers for which . Prove that
Solutions — 3
Solution 1
Let , and ; then gives that . As the numerators of the l.h.s. of the inequality to be proven are positive, the inequality is equivalent to
By expanding, simplifying and using , we get . But this is true, since , and are positive.
Solution 2
Let , and . Then gives that . Therefore there exist positive real numbers , , such that , and .
The inequality can then be written as which is equivalent to
But this inequality can be obtained by adding the obvious inequalities , and .
Solution 3
Let , and ; then gives that . W.l.o.g., let be the greatest of , , . Then , because otherwise , , would all be less than and their product could not be . Thus . Now
Thus , from which the desired inequality can be concluded.
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