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Sum Power Digit Determination (Posted on 2014-07-12) Difficulty: 3 of 5
Determine the last digit of:

12 + 23 + 34 + 45 + .......+ 20142015

No Solution Yet Submitted by K Sengupta    
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Solution Shortcut solution | Comment 2 of 4 |
Any number that end in the digit x is raised to a power that ends in x+1.
The last digit of this number to the power is the same as that for x raised to the same power.

Each digit raised to successive powers has a cycle of length 1, 2 or 4.  (For example the digit 3 has the cycle {3,9,7,1})

So it we look at the first 2000 terms of the sequence above, each last digit, whatever it may be will be included 2000, 1000, or 500 times.  Therefore the last digit of
12 + 23 + 34 + 45 + .......+ 20002001 is zero.
So we really only need the last digits of the final 14 terms.  By closer examination of the cycles I noted above the digits are of these final terms are  1, 8, 1, 4, 5, 6, 1, 8, 1, 0, 1, 2, 9, 4.
The final digit of this sum is 1.

Edited on July 14, 2014, 10:20 am
  Posted by Jer on 2014-07-12 23:51:27

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