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Prove the inequality for the indicated integer values of n

 

( 9/7 ) ^n   >n, n 'greater than or equal to' 9

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 Aug 29, 2016
 #3
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Sorry, for some reason I am unable to see your answer. Would you mind copying and reposting it?

 Aug 29, 2016
 #4
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I deleted my earlier entry because my reasoning was incorrect.  Here is (what I hope is) a correct proof:

 

lhs = (9/7)^n
rhs = n

 

When n = 9  lhs = 9.6...   rhs = 9   so lhs > rhs

 

When n increments by 1 the lhs increments by (9/7)^n(9/7 - 1) = (2/7)*(9/7)^n
The smallest value of this (for n >= 9) is 2.7...,  given when n = 9, and gets larger as n increases.

 

When n increments by 1 the rhs increments by n + 1 - n or just 1 for all n.

 

Hence the lhs starts larger than the rhs (where start is n = 9) and increases more rapidly than the rhs as n increases.

 

i.e. (9/7)^n > n for all n>=9

.

Alan  Aug 29, 2016
 #5
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This is  looks pretty obvious when a graph is plotted:

 

Alan  Aug 29, 2016

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