100 followers problem

Algebra Level 5

x = 4 11 x 9 + 7 x 8 + x 7 69 x 6 64 x 5 + 324 x 4 + 464 x 3 560 x 2 1212 x 320 x 10 + 4 x 9 12 x 8 64 x 7 + 32 x 6 + 384 x 5 + 128 x 4 1024 x 3 768 x 2 + 1024 x + 1024 \sum_{x=4}^{11} \frac{x^9 + 7x^8 + x^7 -69x^6 -64x^5 + 324x^4 +464x^3 -560x^2 -1212x -320}{x^{10}+4x^9 -12x^8 -64x^7 + 32x^6 + 384x^5 + 128x^4 -1024x^3 - 768x^2 + 1024x +1024}

Find the value of the above expression to 4 decimal places.


The answer is 2.1346.

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1 solution

Rahil Sehgal
May 26, 2017

Let S = x = 4 11 x 9 + 7 x 8 + x 7 69 x 6 64 x 5 + 324 x 4 + 464 x 3 560 x 2 1212 x 320 x 10 + 4 x 9 12 x 8 64 x 7 + 32 x 6 + 384 x 5 + 128 x 4 1024 x 3 768 x 2 + 1024 x + 1024 \large S = \displaystyle\sum_{x=4}^{11} \dfrac{x^9 + 7x^8 + x^7 -69x^6 -64x^5 + 324x^4 +464x^3 -560x^2 -1212x -320}{x^{10}+4x^9 -12x^8 -64x^7 + 32x^6 + 384x^5 + 128x^4 -1024x^3 - 768x^2 + 1024x +1024}

This can be written as

x = 4 11 x 9 + 6 x 8 64 x 6 96 x 5 + 192 x 4 + 512 x 3 768 x 512 + ( x 8 16 x 6 + 96 x 4 256 x 2 + 256 ) + ( x 7 + 10 x 6 + 36 x 5 + 40 x 4 80 x 3 288 x 2 380 x 128 ) + ( x 6 4 x 5 4 x 4 + 32 x 3 64 x + 64 16 x 2 ) x 10 + 4 x 9 12 x 8 64 x 7 + 32 x 6 + 384 x 5 + 128 x 4 1024 x 3 768 x 2 + 1024 x + 1024 \large \displaystyle\sum_{x=4}^{11} \dfrac{x^9 + 6x^8 -64 x^6 - 96x^5 + 192x^4 + 512x^3 - 768x - 512 \\ + ( x^8 -16x^6 + 96x^4 -256x^2 + 256) \\ + ( x^7 + 10x^6 + 36x^5 + 40x^4 - 80x^3 - 288x^2- 380x - 128) \\ + ( x^6 - 4x^5 - 4x^4 + 32x^3 -64x + 64 -16x^2) }{x^{10}+4x^9 -12x^8 -64x^7 + 32x^6 + 384x^5 + 128x^4 -1024x^3 - 768x^2 + 1024x +1024}

\\

After factorizing:

S = x = 4 11 ( x + 2 ) 4 ( x 2 ) 3 ( x + 2 ) 2 + ( ( x + 2 ) 4 ( x 2 ) 3 ( x 2 ) ) + ( x + 2 ) 4 ( x + 2 ) 2 ( x 2 ) + ( ( x 2 ) 3 ( x + 2 ) 2 ( x 2 ) ) ( x + 2 ) 4 ( x 2 ) 3 ( x + 2 ) 2 ( x 2 ) \large S = \displaystyle\sum_{x=4}^{11} \dfrac{( x+2)^4 ( x-2)^3 (x+2)^2 \\ + (( x+2)^4 ( x-2)^3 (x-2)) \\ +( x+2)^4 (x+2)^2 (x-2) \\ +( ( x-2)^3 (x+2)^2 (x-2))}{( x+2)^4 ( x-2)^3 (x+2)^2 (x-2)}

\\

This gives us .

S = x = 4 11 1 ( x + 2 ) 4 + 1 ( x 2 ) 3 + 1 ( x + 2 ) 2 + 1 ( x 2 ) \large S = \displaystyle\sum_{x=4}^{11} \dfrac{1}{(x+2)^4} + \dfrac{1}{(x-2)^3} + \dfrac{1}{(x+2)^2} + \dfrac{1}{(x-2)}

\\

On simplifying, we get

1 6 4 + 1 2 3 + 1 6 2 + 1 2 \dfrac{1}{6^4} + \dfrac{1}{2^3} + \dfrac{1}{6^2} + \dfrac{1}{2} \\\cdots \\\cdots \\\cdots \\\cdots

Therefore S S is telescopic and on simplification gives the answer 2.1346 \large \boxed{ \color{#E81990}{\boxed{\color{#3D99F6}{2.1346}}}}

Nice method . But why did u leave so much space?

Aditya Kumar - 3 years, 11 months ago

Did exactly the same way :)

Sahil Silare - 3 years, 3 months ago

I got 2.13426 \boxed{2.13426}

Bob Kadylo - 2 years, 11 months ago

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