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Math Help - Arithmetic Progression

  1. #1
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    Arithmetic Progression

    I have been struggling to understand the question and answer below I came across in my GMAT class.

    Can anyone explain the Q and A below and set another question very similar to this question for me to try and resolve?

    Q1.The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

    A.
    Sol: First term a=2, common difference d=2 since even number

    therefore sum to first n numbers of Arithmetic progression would be

    n/2(2a+(n-1)d)

    = n/2(2*2+(n-1)*2)=n(n+1) and this is equal to 79*80

    therefore n=79 which is odd
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  2. #2
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    Re: Arithmetic Progression

    Quote Originally Posted by KayPee View Post
    I have been struggling to understand the question and answer below I came across in my GMAT class.

    Can anyone explain the Q and A below and set another question very similar to this question for me to try and resolve?

    Q1.The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

    A. Sol: First term a=2, common difference d=2 since even number

    therefore sum to first n numbers of Arithmetic progression would be

    n/2(2a+(n-1)d)

    = n/2(2*2+(n-1)*2)=n(n+1) and this is equal to 79*80

    therefore n=79 which is odd
    What don't you understand in the above solution? And doesn't your textbook have similar questions?
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