Finding closed form for sum of 1.7^k and 2k

[mohabitar]

So here are my steps, which for some reason I feel are very wrong:

Well in closed form would be [n(n+1)]/2 so 2k would be 2*[n(n+1)]/2
For 1.7^k, I used a different form, which I don't have the formula for in front of me, but the final result for that part is [1.7^(n+1) - 1] /[1.7 - 1]

So the final answer I got was [2*[n(n+1)]/2] + [1.7^(n+1) - 1] /[1.7 - 1]

This feels incorrect to me for some reason. Can anyone confirm this or maybe this is correct?

 

[vela]

You can check it yourself. Try a few sums for different values of n and see if the formula you derived gives the same answer. If it does, you can be reasonably sure you have the correct result.

 

[mohabitar]

Well duh I don't know why I didnt think of that. Turns out my answer is correct. Thanks!