MHB Recurrence Relations - Determining a solution of the recurrence relation

AI Thread Summary
The discussion revolves around solving the recurrence relation an = 8an-1 - 16an-2, specifically for proposed solutions involving n, such as an = 2n and an = 4n. The user struggles with plugging in these solutions correctly, particularly how to handle the original sub-values (n-1) and (n-2). It is clarified that for an = 4n, the calculations lead to the correct result, confirming it as a valid solution, while an = 2n does not satisfy the equation. The conversation emphasizes the importance of reviewing properties of exponents to correctly approach these types of problems.
bigpunz04
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Hello -

I am having a tough time understanding the problems in the attached picture (Problem 13). My issue is understanding how I plug in the proposed solutions, specifically those that include n. I am able to solve A and B but unable to solve the rest.

For instance, how do I plug in C or D into the original question? What do I do with the original sub values (n-1) and (n-2) ?

Here is the problem:

"Is the sequence {an} a solution of the recurrence relation
an = 8an-1 - 16an-2 if:"

a) an = 0?
b) an = 1?
c) an = 2n?
d) an = 4n?
e) an = n4n?

My approach to question c

8(2n-1) -16(2n-2)
which I believe gives me...
= 16n-1 - 32n-2

But that answer is obviously wrong. So I'm thinking that I am not plugging the solutions that include n properly. Ugghh so frustrated.
 
Last edited:
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an = 8an-1 - 16an-2

(c)
8(2n-1) -16(2n-2)

note that $2^a(2^b) = 2^{a+b}$ ...

$2^3(2^{n-1}) - 2^4(2^{n-2})$

$2^{n+2} - 2^{n+2} = 0$

use the same property of exponents for (d)
 
skeeter said:
(c)

note that $2^a(2^b) = 2^{a+b}$ ...

$2^3(2^{n-1}) - 2^4(2^{n-2})$

$2^{n+2} - 2^{n+2} = 0$

use the same property of exponents for (d)

Thank you! That definitely gives me an idea and I will now brush up on my laws of exponents. Haven't used it in a while.

Looking at the answers in the back of the book, #c is not a solution for {an} since it is not equal to 2n. It also mentions that #d is a solution since 4n satisfies the equation. However, when I work the equation the same manner as you did, I end of with 0. It should end up with 4n in order to satisfy the equation.

Here is how I worked the problem. You'll notice that I am stuck again.

=8(4n-1) - 16(4n-2)
=23(4n-1) - 24(4n-2)
=23(22+n-1) - 24(22+n-2)
=0
 
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8(4n-1) - 16(4n-2)



$2^3[(2^2)^{n-1}] - 2^4[(2^2)^{n-2}]$

$2^3[2^{2n-2}] - 2^4[2^{2n-4}]$

$2^{2n+1} - 2^{2n}$

$2^{2n}(2 - 1) = 2^{2n}$

recommend you review properties of exponents
 
skeeter said:
$2^3[(2^2)^{n-1}] - 2^4[(2^2)^{n-2}]$

$2^3[2^{2n-2}] - 2^4[2^{2n-4}]$

$2^{2n+1} - 2^{2n}$

$2^{2n}(2 - 1) = 2^{2n}$

recommend you review properties of exponents


I think you are right about reviewing exponential properties. Thank you so much.

Last question...

What happened to the +1 where you wrote 22n+1−22n ?

Thanks again, my friend. I now know exactly where I'm lacking and what I need to review.
 
$2^{2n+1} - 2^{2n}$

$2^{2n} \cdot 2^1 - 2^{2n}$

the two terms above have the common factor $2^{2n}$ ...

$\color{red}{2^{2n}}$ $\cdot 2 - $ $\color{red}{2^{2n}}$

... factor it out from the two terms

$\color{red}{2^{2n}}$ $(2 - 1)$

$2^{2n} (1) = 2^{2n} = 4^n$
 

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