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Algebra Level 5

If x 2 x 1 { x }^{ 2 }-x-1 is a factor of z x 45 + y x 44 + 1 , { zx }^{45}+{ yx }^{44}+1, what is the value of z ? \lvert z\rvert?

Note: x x is a variable, while y y and z z are constants.


The answer is 701408733.

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2 solutions

Mehul Chaturvedi
Dec 27, 2014

Roots of equation are x 2 x 1 { x }^{ 2 }-x-1 are ϕ = 1 + 5 2 , τ = 1 5 2 \phi =\frac { 1+\sqrt { 5 } }{ 2 } ,\tau =\frac { 1-\sqrt { 5 } }{ 2 } .

As x 2 x 1 { x }^{ 2 }-x-1 is factor of z x 45 + y x 44 + 1 { zx }^{ 45 }+{ yx }^{ 44 }+1 .

Therefore we get z ϕ 45 + y ϕ 44 + 1 = 0 , z τ 45 + y τ 44 + 1 = 0 z\phi ^{ 45 }+{ y\phi }^{ 44 }+1=0,{ z\tau }^{ 45 }+{ y\tau }^{ 44 }+1=0

Subtracting 2nd equation from 1st we get z ϕ 45 z τ 44 + y ϕ 44 y τ 45 = 0 = z ( ϕ 45 τ 45 ) + y ( ϕ 44 τ 44 ) z\phi ^{ 45 }-{ z\tau }^{ 44 }+{ y\phi }^{ 44 }-{ y\tau }^{ 45 }=0=z(\phi ^{ 45 }-{ \tau }^{ 45 })+{ y(\phi }^{ 44}-{ \tau }^{ 44 })

Now we have formula f n = ( ϕ n τ n ) 5 { f }_{ n }=\frac { \left( { \phi }^{ n }-{ \tau }^{ n } \right) }{ \sqrt { 5 } } where f n { f }_{ n } is n t h nth Fibonacci number.

Using above formula we have our expression as z 5 f 45 + y 5 f 44 = 0 z{ \sqrt { 5 } f }_{ 45 }+y{ \sqrt { 5 } f }_{ 44 }=0 .,,,,,(1)

Dividing by 5 \sqrt { 5 } we get z f 45 + y f 44 = 0 z{ f }_{ 45 }+y{ f }_{ 44 }=0 .

As we know 45 t h 45th and 44 t h 44th Fibonacci numbers are 1134903170 1134903170 and 701408733 701408733 .

Now substituting in (1) we get our equation as 1134903170 z + 701408733 y = 0 1134903170z+701408733y=0

Clearly z = ± 701408733 n , y = 1134903170 n z=\pm 701408733n,y=\mp 1134903170n for n = 0 , 1 , 2 , . . . . n=0,1,2,....

Our answer occur's at n = 1 n=1 and z = ± 701408733 z=\pm 701408733

So our answer is 701408733 \Huge\boxed{701408733} .

Why only n=1 ?

Aneesh Kundu - 6 years, 5 months ago

how do you get 45th fibonacci number ??

Chirayu Bhardwaj - 5 years ago
Vijay Bhaskar
Jun 6, 2015

z x^45+y x^44+1=(x^2-x-1)(a43x^43+a42x^42+...+a1x+a0)

Hence,

a0=-1
-a0-a1=0 => a1=1
a0-a1-a2=0 => a2=a0-a1 = -1
a1-a2-a3=0 => a3=a1-a2 = 2
a2-a3-a4=0 => a4=a2-a3 = -3
....
a41-a42-a43=0 => a43=a41-a42=(-1)^44*fibonacci(43)



Now, y = a42-a43, z=a43

Hence abs(z) = abs(a43) = fibonacci(43) = 701408733

Moderator note:

To start a new line, leave 3 empty spaces at the end of the line. I've edited your solution and it's now much easier to understand what you're trying to say.

Good job with spotting the pattern, though you should also explain why it's true (induction would be one approach).

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