A perfectly natural question ....

Calculus Level 5

It is the case that

n = 1 1 2 n ( n 1 2 ) = a ln ( b + c ) \displaystyle\sum_{n=1}^{\infty} \dfrac{1}{2^{n} * (n - \frac{1}{2})} = \sqrt{a}*\ln(b + \sqrt{c}) ,

where a , b , c a,b,c are positive integers with a a and c c being square-free.

Find a b + c a^{b+c} .


The answer is 8.

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3 solutions

Akshay Bodhare
Jan 30, 2015

S = n = 1 1 2 n ( n 1 2 ) = 2 n = 1 ( 1 2 ) ( 2 n 1 ) ( 2 n 1 ) S=\displaystyle\sum_{n=1}^{\infty} \frac{1}{2^{n}*(n-\frac{1}{2})} = \sqrt{2}\displaystyle\sum_{n=1}^{\infty} \frac {(\dfrac {1}{\sqrt{2}})^{(2n-1)}}{(2n-1)}

Now,we know that,

log ( 1 + x ) = n = 1 ( 1 ) ( n 1 ) x n n \log {(1+x)} = \displaystyle\sum_{n=1}^{\infty}(-1)^{(n-1)} \frac {x^n}{n}

Also,

log ( 1 x ) = n = 1 x n n -\log{(1-x)} =\displaystyle\sum_{n=1}^{\infty}\frac{x^n}{n}

Adding both gives us,

log ( 1 + x 1 x ) 1 2 = n = 1 x ( 2 n 1 ) 2 n 1 \log{(\frac {1+x}{1-x})}*{\frac{1}{2}}=\displaystyle\sum_{n=1}^{\infty}\frac{x^{(2n-1)}}{2n-1}

Therefore,

S = 2 n = 1 ( 1 2 ) ( 2 n 1 ) ( 2 n 1 ) = 1 2 2 log ( 1 + 1 2 1 1 2 ) S=\sqrt{2}\displaystyle\sum_{n=1}^{\infty} \frac {(\dfrac {1}{\sqrt{2}})^{(2n-1)}}{(2n-1)} = \frac {1}{2} \sqrt {2} {\log (\frac {1+\frac {1}{\sqrt{2}}}{1-\frac {1}{\sqrt{2}}})}

Rationalizing,the radical,we get,

S = 1 2 2 log ( 1 + 2 ) 2 = 2 log ( 1 + 2 ) S= \frac {1}{2} \sqrt {2} {\log {(1+\sqrt{2})}^2}=\sqrt {2} {\log {(1+\sqrt{2})}}

Excellent! This is such an elegant solution, Akshay. Thank you for posting it. :)

Brian Charlesworth - 6 years, 4 months ago

Did the same! Nice question! Also try this

Kartik Sharma - 6 years, 4 months ago

The sum is 2 ln ( 1 + 2 ) \sqrt{2}*\ln(1 + \sqrt{2}) , making a b + c = 2 1 + 2 = 8 a^{b+c} = 2^{1+2} = \boxed{8} .

I debated whether to post this as a question or a note, since I'm still working on a proof. It's such a beautiful result, though, that I wanted to get it posted regardless and see if the community has any ideas on an elegant proof.

All I could think of is integrating n = 1 x n 3 / 2 = 1 x ( 1 x ) \displaystyle\sum_{n=1}^{\infty} x^{n-3/2}=\dfrac{1}{\sqrt{x}(1-x)} by using x x \sqrt{x} \rightarrow x

Pratik Shastri - 6 years, 4 months ago

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I tried using your approach,I am getting the correct answer but how do we find the constant term of integration,we get the answer only on assuming that the constant term is zero.

Akshay Bodhare - 6 years, 4 months ago

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Put x = 0 x=0 .

Pratik Shastri - 6 years, 4 months ago

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@Pratik Shastri First term of The series found becomes \infty .

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare Sorry,understood my mistake.

Akshay Bodhare - 6 years, 4 months ago

Yes, that works perfectly, and it doesn't require remembering any series representations. Thanks! :)

Brian Charlesworth - 6 years, 4 months ago

Can you elaborate on how you got this ?

Thanks for the same .

A Former Brilliant Member - 6 years, 4 months ago

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n = 1 x n = x 1 x \displaystyle\sum_{n=1}^{\infty} x^n = \dfrac {x}{1-x} using G.P for x < 1 |x| < 1

n = 1 x n 3 2 = 1 x ( 1 x ) \displaystyle\sum_{n=1}^{\infty} x^{n-\frac{3}{2}} = \dfrac {1}{\sqrt{x}(1-x)}

Now ,integrating we get,

log ( 1 + x 1 x ) = n = 1 x n 1 2 n 1 2 \log {(\frac{1+\sqrt{x}}{1-\sqrt{x}})}=\displaystyle\sum_{n=1}^{\infty} \frac{x^{n-\frac{1}{2}}}{n-\frac{1}{2}}

And then we put x = 1 2 x=\frac{1}{2}

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare Thanks Akshay Bodhare ,actually I was wondering about where's the 2 n 2^{n} vanished to , so I got confused

A Former Brilliant Member - 6 years, 4 months ago

I used the Taylor expansion of arctanh(sqrt(x))/sqrt(x) to get the result and after substituting x=2 we ge the desired result

Oussama Boussif - 6 years, 4 months ago

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Great. That's similar to Akshay's posted solution, except that he started with the series for the log function and essentially derived the series for the arctanh function.

Brian Charlesworth - 6 years, 4 months ago
Incredible Mind
Jan 31, 2015

i used integrals on (n-1/2) via 1/A = integral 0 to inf e^(-Ax)dx , A>0

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