What is the limit of 1/n^2 + 2/n^2 + + n-1/n^2 as n-> infinity

In summary, The limit of the given expression as n approaches infinity is equal to 1/2. This can be found by using the summation formula for arithmetic series or by viewing the problem as a Riemann sum for the integral of f(x)=x from x=0 to x=1. The initial guess of the limit being zero was incorrect.
  • #1
sara_87
763
0

Homework Statement



find:

lim(n[tex]\rightarrow\infty[/tex] (1/n^2 + 2/n^2 + 3/n^2 + ... + n-1/n^2 )

Homework Equations





3. The Attempt at a Solution [/b

I could guess that the limit is zero but i don't know howto prove it
 
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  • #2


sara_87 said:

Homework Statement



find:

lim(n[tex]\rightarrow\infty[/tex] (1/n^2 + 2/n^2 + 3/n^2 + ... + n-1/n^2 )

Homework Equations





3. The Attempt at a Solution [/b

I could guess that the limit is zero but i don't know howto prove it

No, the limit is not 0.

That sum is the same as (1/n2)(1+ 2+ 3+ ...+ (n-1)). Can you write that last sum in closed form?
 
  • #3


Oh right,
so, this is the same as:

[tex]\frac{1}{n^2}[/tex][tex]\sumk[/tex] (from k=1 to n-1)

and now i use the formula for geometric series?
 
  • #4


Dick asked whether you knew a formula for 1 + 2 + 3 + ... + (n - 1). This is not a geometric series.
 
  • #5


What is meant by 'closed form' ? i don't know, i thought dick meant that i should put it in a summation.
i meant arithmetic...sorry, surely this is not geometric.
 
  • #6


I have done it and i just want to make sure of my answer:
i used the summation formula for aritmetic series :1/2 n(n+1)
and got that the limit is 1/2
is that right?
 
  • #7


sara_87 said:
I have done it and i just want to make sure of my answer:
i used the summation formula for aritmetic series :1/2 n(n+1)
and got that the limit is 1/2
is that right?

Sure it's right. You could also look at the problem as being a Riemann sum for the integral of f(x)=x from x=0 to x=1. Surely, 1/2. BTW, I didn't say anything before in this thread. That was Halls. Let's give credit where credit is due.
 
  • #8


Dick said:
BTW, I didn't say anything before in this thread. That was Halls. Let's give credit where credit is due.

Sorry for giving credit where not due. I think I saw your name in another thread that sara started, and mistakenly cited you and Dirk.
 
  • #9


Mark44 said:
Sorry for giving credit where not due. I think I saw your name in another thread that sara started, and mistakenly cited you and Dirk.

No problem. If Hall's suggestion had been wrong, I would have been OUTRAGED to have it attributed to me. But it wasn't. :)
 

Related to What is the limit of 1/n^2 + 2/n^2 + + n-1/n^2 as n-> infinity

1. What is the significance of the expression 1/n^2 + 2/n^2 + ... + n-1/n^2?

The expression represents a mathematical series that converges to a finite value as n approaches infinity. This means that as n increases, the sum of the terms in the series approaches a limit value.

2. How do you calculate the limit of 1/n^2 + 2/n^2 + ... + n-1/n^2 as n approaches infinity?

To calculate the limit, we can use the formula for the sum of a finite geometric series: S = a(1-r^n)/(1-r), where a is the first term, r is the common ratio, and n is the number of terms. In this case, a = 1, r = 1/n, and n = n-1. Plugging in these values and taking the limit as n approaches infinity, we get S = 1/(1-1/n) - 1 = 1 - 1 = 0.

3. What is the significance of the limit of 1/n^2 + 2/n^2 + ... + n-1/n^2 as n-> infinity?

The limit represents the end behavior of the series as n approaches infinity. It tells us the value that the series will approach and helps us understand the overall behavior of the series.

4. Is it possible for the limit of 1/n^2 + 2/n^2 + ... + n-1/n^2 to be a negative value?

No, the limit of the series will always be a positive value. This is because as n increases, the terms in the series become smaller and smaller, approaching zero. Therefore, the sum of these terms will also approach zero, which is a positive value.

5. How does the limit of 1/n^2 + 2/n^2 + ... + n-1/n^2 as n-> infinity compare to other infinite series?

The limit of this series, 0, is considered a convergent limit, meaning that the series approaches a finite value as n-> infinity. This is in contrast to a divergent limit, where the series does not approach a finite value. In comparison to other infinite series, this series converges faster than many other commonly studied series, such as the harmonic series.

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