Question 218807: What is the units digit of 3^1 + 3^3 + 3^5 + 3^7 +………+ 3^2009 ?
Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website! What is the units digit of 3^1 + 3^3 + 3^5 + 3^7 +………+ 3^2009 ?
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The units digit for powers of 3 is cyclic. It goes:
3, 9, 7, 1, 3, 9, 7, 1 ...
The sequence above is 3,7,3,7, etc Each pair adds to 0.
2009 modulo 4 = 1
The units digit of the sum is 3.
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