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The Last Two Digits (
Posted on 20130904
)
What are the last two digits of (1+5^(2n+1))/6?
Note that n is assumed to be any positive integer.
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Submitted by
Danish Ahmed Khan
Rating:
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(1 votes)
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another way
 Comment 2 of 3 
Factor the numerator as (1+5)(15+...+5^2n) and note that positive powers ending in 25 are offset by negatives. You're left with (15+25)=21 since the (1+5) factor cancels.
Posted by
xdog
on 20130904 12:22:43
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