How to solve convergences when dealing with series

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In summary, the sequence U(n+1) = \sqrt{2U(n) + 5} where U(1) = 3 converges to a limit, u, and find the value of u correct to 2 decimal places.
  • #1
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Homework Statement



shows that the sequence [tex]U(n+1) = \sqrt{2U(n) + 5}[/tex] where U(1) = 3 converges to a limit, u, and find the value of u correct to 2 decimal places.

Homework Equations





The Attempt at a Solution



I only know how to solve convergences when dealing with series lol.

Any help please :)

thanks
 
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  • #2
If the sequence does have a limit L, then the limit must satisfy L=sqrt(2L+5). Do you see why? As for the proof part can you show the sequence is increasing and bounded?
 
  • #3
ooooo

yeah I see why L=sqrt(2L+5).

I still can't go anywhere from here though :S
 
  • #4
Ok. Take the function f(x)=sqrt(2x+5). You get the elements in the sequence by finding f(3), f(f(3)), f(f(f(3))) etc. To show that f(x)>x (that the sequence is increasing), you want to show f(x)-x>0. For what values of x is this true?
 
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  • #5
Trail_Builder said:
ooooo

yeah I see why L=sqrt(2L+5).

I still can't go anywhere from here though :S
You can't solve that for L? L2= 2L+ 5 so L2- 2L= 5. You can solve that by completing the square: L2- 2L+ 1= 5+ 1= 6. (L- 1)2= 6 so [itex]L- 1= \pm\sqrt{6}[/itex] and [itex]L=1\pm\sqrt{6}[/itex]. Here, since the sequence starts at 3 and increases, [itex]L= 1+ \sqrt{6}[/itex]. Of course, we need to prove that the sequnce is increasing and bounded above so it will converge.

Prove U(n)< U(n+1) for all n, by induction. If n= 1, then U(1)= 3 while U(2)= [itex]2\sqrt{2U(1)+5}= \sqrt{6+ 5}=\sqrt{11}> 3[/itex] (because 11> 9).
Now, suppose for some k, U(k+1)> U(k). Then U((k+1)+ 1)= [itex]\sqrt{2U(k+1)+ 5}> \sqrt{2U(k)+ 5}= U(k+1)[/itex].

You will also need to prove that the sequence is bounded above. Since (if the sequence has a limit) the limit is [itex]1+ \sqrt{6}[/itex] which is approximately 3.5so you might try proving, again by induction, that U(n)< 4 for all n.
 
  • #6
o sick

thanks for help guys :D

much appreciated

I see it now :)
 

FAQ: How to solve convergences when dealing with series

What is a convergence?

A convergence is a mathematical concept that refers to the behavior of a sequence or series as its terms approach a certain value. In other words, it is the tendency of a sequence or series to approach a certain limit.

What is the difference between convergence and divergence?

Convergence refers to the behavior of a sequence or series as its terms approach a limit, while divergence refers to the behavior of a sequence or series as its terms become infinitely large or small.

How do you determine if a series is convergent or divergent?

The most common way to determine the convergence or divergence of a series is by using mathematical tests, such as the Comparison Test, Ratio Test, or Integral Test. These tests involve evaluating the behavior of the terms in the series and can help determine if the series is convergent or divergent.

What is the purpose of solving for convergence when dealing with series?

Solving for convergence allows us to determine the behavior of a series and whether or not it will approach a certain limit. This is important in many areas of mathematics, as it helps us understand the behavior of infinite sequences and series and their applications.

What are some common techniques used to solve convergences when dealing with series?

Some common techniques used to solve convergences in series include algebraic manipulation, using mathematical tests, and using properties of limits. It is also important to understand the behavior and patterns of different types of series, such as geometric series or power series, in order to effectively solve for convergence.

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