Few questions about deriving expressions to find the runtime using summations. The Big-Oh time complexity is already given, so using summations to find the complexity is what I am focused on.

So I know that there are 2 instructions that must be run before the first iteration of the loop, and 2 instructions that have to be run, (the comparison, and increment of i) after the first iteration. Of course, there is only 1 instruction within the for loop. So deriving I have 2n + 3, ridding of the 3 and the 2, I know the time complexity is O(n).

Here I know how to start writing the summation, but the increment in the for loop is still a little confusing for me. Here is what I have:

So I know my summation time complexity derivation is wrong. Any ideas as to where I'm going wrong? Thank you