Verified answer

Hello.

I'd like to point out a pattern so that you may finish the problem yourself.

3^1 = 3

3^3 = 27

3^5 = 243

3^7 = 2187

You can see that the units digit of each term in the series alternates between 3 and 7. So, to find the sum of the series 3^1 + 3^3 + ... + 3^2009, you need to sum up the units digits of each term and look at the units digit of the resulting number - that will be your answer.

Give it a try.

Good luck.