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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

    ehild

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    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.
     
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