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Exact upper and lower limit of the sequence

wishmaster

Active member
Oct 11, 2013
211
I have one more problem,for the given sequence i have to find the exact upper and lower limit,and to argument them. i have been missing on this lesson,so please help me,i dont know how to do it.

So the sequence is:

an = \(\displaystyle \frac{2n+3}{n}\)
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
If I am interpreting correctly what you meant by upper and lower limit, I would rewrite the $n$th term as follows:

\(\displaystyle a_n=2+\frac{3}{n}\)

What can you say about the terms as $n$ grows?
 

wishmaster

Active member
Oct 11, 2013
211
If I am interpreting correctly what you meant by upper and lower limit, I would rewrite the $n$th term as follows:

\(\displaystyle a_n=2+\frac{3}{n}\)

What can you say about the terms as $n$ grows?
I would say that upper limit is 5 and lower limit is 2.
But how to prove it? Or show it in math way?
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
How did you determine the bounds?
 

wishmaster

Active member
Oct 11, 2013
211

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
I have assumed for a1
You know:

\(\displaystyle a_1=2+\frac{3}{1}=5\)

and you know that \(\displaystyle \frac{3}{n}\) gets smaller as $n$ increases, so then you know $a_n$ is monotonically decreasing. How can you determine the lower bound?
 

wishmaster

Active member
Oct 11, 2013
211
You know:

\(\displaystyle a_1=2+\frac{3}{1}=5\)

and you know that \(\displaystyle \frac{3}{n}\) gets smaller as $n$ increases, so then you know $a_n$ is monotonically decreasing. How can you determine the lower bound?
\(\displaystyle \lim _{n \to \infty}2+ \frac{3}{n} = 2 + 0=0\) ???
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775

wishmaster

Active member
Oct 11, 2013
211
Correct, except that $2+0=2$.

Im sorry,thats what i thought..my mistake.

So with limit i have proved exact lower limit,i know that exact upper limit is 5,but how to write it correctly? With proof.....
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
Im sorry,thats what i thought..my mistake.

So with limit i have proved exact lower limit,i know that exact upper limit is 5,but how to write it correctly? With proof.....
What more do you need? You know the sequence bounds and that it is monotonic. All you need to do is discuss the fact that $\dfrac{3}{n}$ decreases as $n$ increases. Or you could consider the difference:

\(\displaystyle \frac{3}{n+1}-\frac{3}{n}\)

And show that it is negative for all $n\in\mathbb{N}$.