Consecutive odd integers

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Ramalakshmi: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat Jan 15, 2011 9:14 pm

by Ramalakshmi » Sat Jan 15, 2011 9:16 pm

Find n if n is the sum of consecutive odd numbers whose sum is 81?

Can anyone help me out with this problem

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Source: Beat The GMAT — Problem Solving | Sat Jan 15, 2011 9:16 pm

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by Anurag@Gurome » Sat Jan 15, 2011 9:20 pm

Ramalakshmi wrote:Find n if n is the sum of consecutive odd numbers whose sum is 81?

Please check the question.

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anshumishra: Legendary Member; Posts: 543; Joined: Tue Jun 15, 2010 7:01 pmFollowed by: 3 members; Thanked: 147 times

by anshumishra » Sat Jan 15, 2011 9:26 pm

Ramalakshmi wrote:Find n if n is the sum of consecutive odd numbers whose sum is 81?

Can anyone help me out with this problem

As Anurag has suggested check the problem, it is not properly written.
If the consecutive numbers start from 1, means the series is something like (1,3,5...), then

Sum of n terms = n^2 = 81
=> n = 9.

BUT, if the question is different , please update it.
You may use this formula :

Sum = n/2[2a+(n-1)*2], where "a" is the first term

Thanks
Anshu

(Every mistake is a lesson learned )

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Ramalakshmi: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat Jan 15, 2011 9:14 pm

by Ramalakshmi » Sat Jan 15, 2011 9:41 pm

The question is correct .....

Find n if n is the sum of consecutive odd positive numbers whose sum is 81?

n^2=81

9 is the answer

Thanks

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sat Jan 15, 2011 10:07 pm

Ramalakshmi wrote:The question is correct .....

Find n if n is the sum of consecutive odd positive numbers whose sum is 81?

There is no way the question is correct.
If "n is the sum of consecutive odd positive numbers whose sum is 81, then the value of n must be 81 itself!

If 9 is the answer then the proper question should be something like : Find n if the sum of first n consecutive odd positive integers is 81.

In that case, the integers are 1, 3, 5, ..., (2n - 1)
The sum = n*[ 1 + (2n - 1)]/2 = n^2

=> n^2 = 8
=> n = 9

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