Line having power nine

Algebra Level 3

r = 1 511 y r 9 = r = 1 512 x ( 2 r 1 ) 9 \displaystyle{ \sum_{r=1}^\infty \dfrac{511 y}{r^9}} ={ \sum_{r=1}^\infty \dfrac{512 x}{ (2r-1)^9}} represents a line. Find the slope of this line.


The answer is 1.

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1 solution

Rishabh Jain
Jun 4, 2016

Let I = r = 1 1 r 9 \mathcal I=\displaystyle{ \sum_{r=1}^\infty \dfrac{1 }{r^9}} and S = r = 1 1 ( 2 r 1 ) 9 \displaystyle\mathcal S={ \sum_{r=1}^\infty \dfrac{1}{ (2r-1)^9}}

Then the equation of given line can be written in the form: y = 512 S 511 I x y=\color{#3D99F6}{\dfrac{512~\mathcal S}{511~\mathcal I}}x

Now, I = r = 1 1 r 9 = r = 1 1 ( 2 r 1 ) 9 S + r = 1 1 ( 2 r ) 9 1 2 9 ( I ) = I 512 \mathcal{I}=\displaystyle{ \sum_{r=1}^\infty \dfrac{1 }{r^9}} = \underbrace{\displaystyle{ \sum_{r=1}^\infty \dfrac{1}{ (2r-1)^9}}}_{\large\mathcal S}+\underbrace{\displaystyle{ \sum_{r=1}^\infty \frac{1 }{(2r)^9}}}_{\large\frac{1}{2^9}(\mathcal{I})=\frac{\mathcal I}{512}} 511 I 512 = S \large\implies \dfrac{511~\mathcal I}{512}=\mathcal{ S}

512 S 511 I = 1 \large \implies \color{#3D99F6}{\dfrac{512~\mathcal S}{511~\mathcal I}}=\huge\boxed{\color{#D61F06}{1}}

(which is obviously the slope of given line)

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