1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Finding the first 40 positive odd integers

  1. Jun 16, 2016 #1
    I have the following problem: What is the sum of the first 40 positive odd integers?

    I look at the solution, and it says that "The sum of the first 40 positive odd integers is ##1 + 3 + 5 + \dotsm + 77 + 79##. And then it goes on with the solution.

    My question is, how do I find that 79 is the 40th positive odd integer? Do I have to just write them all down until I get to the 40th one?
  2. jcsd
  3. Jun 16, 2016 #2


    User Avatar
    Homework Helper
    Gold Member

    The first odd positive integer is 1. The second one is 1+2. The third one is 1+2+2. How many times you need to add 2 to get 40 terms?
    (What kind of sequence is 1, 3, 5. 7, ... ? What is the N-th term of that sequence?)
  4. Jun 17, 2016 #3
    Ah, so I can just create an arithmetic sequence ##1 + 2(n - 1)##, where n is the index of the odd number. So when I substitute 40 I just get 1 + 2(39) = 79! Awesome, thanks.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Finding the first 40 positive odd integers