I need help writing equations please...

[Alpha6]

So I had to solve the following problem...

X_n+1 - 5X_n + 4X_n-1 = 0 X₁ = 9 and X₂ = 33

So I set X = 1 and got:

X₁₊₁ - 5X₁ + 4X_1-1 = 0

(33) - 5(9) + 4X₀ = 0

33-45 + 4X₀ = 0

-12 + 4X₀ = 0

4X₀ = 12

X₀ = 3

So we know that X₀= 3, X₁=9, and X₂=33

Knowing this, how do I write the equation?? The equation is X_n = 1 + 2 * 4ⁿ However, I have no idea how to arrive at that equation if the book did not tell me. Any help please? There are many problems like these, I can always solve for X but I can't find the general equation.

 

[Brainwave]

solution

You would have been thought difference equation for this type of question. You can see see my solution in the attachment below

 

[Alpha6]

Thanks for replying.

I see why C₁=2 and C₂=1 but I wouldn't know how to work that out.

Do you use different numbers until one works out? A trial and error approach?

 

[JeffM]

So I had to solve the following problem...

X_n+1 - 5X_n + 4X_n-1 = 0 X₁ = 9 and X₂ = 33

So I set X = 1 and got:

X₁₊₁ - 5X₁ + 4X_1-1 = 0

(33) - 5(9) + 4X₀ = 0

33-45 + 4X₀ = 0

-12 + 4X₀ = 0

4X₀ = 12

X₀ = 3

So we know that X₀= 3, X₁=9, and X₂=33

Knowing this, how do I write the equation?? The equation is X_n = 1 + 2 * 4ⁿ However, I have no idea how to arrive at that equation if the book did not tell me. Any help please? There are many problems like these, I can always solve for X but I can't find the general equation.

You can virtually "see" this one if you already know the powers of 2: 3 is 1 greater than 2, 9 is 1 greater than 8, and 33 is one greater than 32, and 2, 8, and 32 are the successive odd powers of 2.

So $\displaystyle x_n = 1 + 2^{(2n - 1)} = 1 + 2 * 2^{2n} = 1 + 2 * 4^n.$

But perhaps you do not "see" that the powers of 2 are involved. What next?

If successive terms differ by a constant factor, you can discover the common factor as follows: $\displaystyle x_{n + 1} = a * x^n \ne 0 \implies \dfrac{x_{n + 1}}{x_n} = a.$

That does not work here because 9 / 3 = 3 and 33 / 9 > 3.

If successive terms differ by a constant, you can find the constant as follows: $\displaystyle x_{n + 1} = x_n + a \implies a = x_{n + 1} - x_n.$

That does not work here because 9 - 3 = 6 and 33 - 9 = 24. But maybe there is a ratio between these differences. 24 / 6 = 4.

We need more examples to see if there is a pattern.

$\displaystyle x_3 - 5x_2 + 4x_1 = 0 \implies x_3 - 5 * 33 + 4 * 9 = 0 \implies x_3 = 165 - 36 = 129.$

$\displaystyle x_4 - 5x_3 + 4x_2 = 0 \implies x_3 - 5 * 129 + 4 * 33 = 0 \implies x_4 = 645 - 132 = 513.$

So now the successive differences are 6, 24, 129 - 33 = 96, and 513 - 129 = 384.

But what are the ratios of the successive differences 24 / 6 = 4, 96 / 24 = 4, 384 / 96 = 4.

OK, it looks as though 4 is involved as follows:

$\displaystyle \dfrac{x_{n + 1} - x_n}{x_n - x_{n - 1}} = 4 \implies x_{n + 1} - x_n = 4x_n - 4x_{n - 1} \implies x_{n + 1} - 5x_n + 4x_{n - 1} = 0.$ We are definitely on the right track.

In fact, 24 / 6 = 4, 96 / 6 = 16, 384 / 6 = 64. It looks as though powers of 4 are involved.

$\displaystyle x_1 = 9 = 2 * 4 + 1 = 1 + 2 * 4^1.$

$\displaystyle x_2 = 33 = 2 * 16 + 1 = 1 + 2 * 4^2.$

$\displaystyle x_3 = 129 = 2 * 64 + 1 = 1 + 2 * 4^3.$

$\displaystyle x_4 = 513 = 2 * 256 + 1 = 1 + 2 * 4^4.$

This looks very promising. If this pattern works $\displaystyle 1 + 2 * 4^0 = 1 + 2 * 1 = 1 + 2 = 3 = x_0.$

Now lets test.

$\displaystyle x_n = 1 + 2 * 4^n \implies x_{n + 1} - 5x_n + 4x_{n - 1} = 1 + 2 * 4^{(n+1)} - 5\left(1 + 2 * 4^n\right) + 4(1 + 2 * 4^{n-1)}) =$

$\displaystyle 1 + 2 * 4^{(n + 1)} - 5 - 10 * 4^n + 4 + 8 * 4^{(n - 1)} =$

$\displaystyle 2 * 4^{(n + 1)} - 10 * 4^n + 8 * 4^{(n - 1)} =$

$\displaystyle 2 * 4^2 * 4^{(n - 1)} - 10 * 4^1 * 4^{(n-1)} + 8 * 4^{(n - 1)} =$

$\displaystyle 4^{(n - 1)}(2 * 4^2 - 10 * 4^1 + 8) = 4^{(n - 1)}(2 * 16 - 10 * 4 + 8) = 4^{(n - 1)}(32 - 40 + 8) = 4^{(n - 1)} * 0 = 0.$

I do not know a general method for finding a closed form for a recursively defined function. I generally work out a number of terms of the recursion and then start taking differences, ratios, ratios of differences until I find some consistent element and then start trying to use that consistent element as part of a pattern. If you take this experimental approach, you must then test to make sure that the pattern is not spurious.

 

[Brainwave]

Try to check textbooks for reference. But how I got C1 and C2 is below.

 

[HallsofIvy]

So I had to solve the following problem...

X_n+1 - 5X_n + 4X_n-1 = 0 X₁ = 9 and X₂ = 33

So I set X = 1 and got:

What? There is no "X", without a subscript in the problem!
Oh,I see, you mean you set n equal to 1.

X₁₊₁ - 5X₁ + 4X_1-1 = 0

(33) - 5(9) + 4X₀ = 0

Did this sequence start with $\displaystyle X_0$ rather than $\displaystyle X_1$? That's possible but if so, odd that the gave $\displaystyle X_1$ and $\displaystyle X_2$ as the initial values rather than $\displaystyle X_0$ and $\displaystyle X_1$.

33-45 + 4X₀ = 0

-12 + 4X₀ = 0

4X₀ = 12

X₀ = 3

So we know that X₀= 3, X₁=9, and X₂=33

Knowing this, how do I write the equation?? The equation is X_n = 1 + 2 * 4ⁿ However, I have no idea how to arrive at that equation if the book did not tell me. Any help please? There are many problems like these, I can always solve for X but I can't find the general equation.

If you are asking "What rule can I follow or what calculation can I do to get this without having to think", it just doesn't work that way. Instead, calculate some more values and look for a pattern! Knowing that $\displaystyle X_1= 9$ and $\displaystyle X_2= 33$,
$\displaystyle X_{n+1}- 5X_n+ 4X_{n-1}= 0$
With n= 2 $\displaystyle X_3- 5X_2+ 4X1= X_3- 5(33)+ 4(9)= X_3- 129= 0$ so $\displaystyle X_3= 129$.
With n= 3 $\displaystyle X_4- 5X_3+ 4X_2= X_4- 5(129)+ 4(33)= X_4- 513$ so $\displaystyle X_4= 513$.
With n= 4 $\displaystyle X_5- 5X_4+ 4X_2= X_5- 5(513)+ 4(129)= X_5- 2049$ so $\displaystyle X_5= 2049$.

Continue if you like but eventually you should recognize that 9= 1+ 8, 33= 1+ 32, 129= 1+ 128, 513= 1+ 512, and 2049= 1+ 2048.

That is, every number is 1 plus a power of 2: $\displaystyle 8= 2^3$, $\displaystyle 32= 2^5$, $\displaystyle 128= 2^{7}$, $\displaystyle 512= 2^9$, and \(\displaystyle 2048= 2^11\(\displaystyle . Noting further that those are all odd powers of 2, I can write them as $\displaystyle 2^3= 2^1(2^2)= 2(4)$, $\displaystyle 2^5= 2^1(2^2)(2^2)= 2(4^2)$, $\displaystyle 2^7= 2^1(2^2)(2^2)(2^2)= 2(4^3)$, $\displaystyle 2^9= 2^1(2^2)(2^2)(2^2)(2^2)= 2(4^4)$, and $\displaystyle 2^{11}= 2^1(2^2)(2^2)(2^2)(2^2)(2^2)= 2(4^5)$. Do you see I now?\)\)