Homework inequality -- Show that (a+1)(b+1)(c+1)(d+1) < 8(abcd+1)

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SUMMARY

The inequality (a+1)(b+1)(c+1)(d+1) < 8(abcd+1) for a, b, c, d > 1 can be approached using the AM-GM inequality and induction. Initial attempts to prove the inequality for two variables were successful, showing that (a+1)(b+1) < 2(ab+1). The discussion highlights the necessity of recognizing that ab > 1 and cd > 1 to facilitate the proof. The collaborative effort among forum members has provided a pathway to complete the proof using these insights.

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Homework Statement


For a,b,c,d >1, Show that (a+1)(b+1)(c+1)(d+1) < 8(abcd+1)

Homework Equations


How to show this?

The Attempt at a Solution


I could show for two variables, (a+1)(b+1)<2(ab+1). Tried C-S, AM-GM inequalities in different form and variable transformations. But still no result. It's my daughters test question. Any help is appreciated.
 
Last edited:
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Can this be done by induction?
 
(a+1)(b+1)<2(ab+1)
(d+1)(c+1)<2(dc+1)

(a+1)(b+1)(d+1)(c+1)< 4(ab+1)(dc+1)

Can you complete now ?
 
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ssd, Buffu has given a nice lead. Recognize that ##ab>1## and ##cd>1##. So replace these in your expressions for ##a## and ##b##
 
Thanks a lot to both of you.
 

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