Are you smarter than me? 4

Calculus Level 4

1 n + 9 ( 2 n + 9 ) ( n ) ( n + 8 ) \sum _{ 1 }^{ \infty }{ \frac { n+9 }{ { (2 }^{ n+9 })(n)(n+8) } }

State your answer to 5 decimal places.


The answer is 0.001498.

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2 solutions

Kartik Sharma
Nov 26, 2014

n = 1 n + 9 ( 2 n + 9 ) ( n ) ( n + 8 ) \sum _{ n=1 }^{ \infty }{ \frac { n+9 }{ { (2 }^{ n+9 })(n)(n+8) } }

= n = 1 1 ( 2 n + 9 ) × n + 9 n ( n + 8 ) \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { (2 }^{ n+9 }) } } \times \frac { n+9 }{ n(n+8) }

= n = 1 1 ( 2 n + 9 ) ( 9 8 ( n ) + 1 8 ( n + 8 ) ) \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { (2 }^{ n+9 }) } } \left( \frac { 9 }{ 8(n) } +\frac { 1 }{ 8(n+8) } \right)

= n = 1 1 ( 2 n + 12 ) ( 9 n ) + n = 1 1 ( 2 n + 12 ) ( 1 n + 8 ) \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { (2 }^{ n+12 }) } \left( \frac { 9 }{ n } \right) } \quad +\quad \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { (2 }^{ n+12 }) } \left( \frac { 1 }{ n+8 } \right) }

= 9 2 12 ( n = 1 1 2 n ( n ) ) + 1 2 12 ( n = 1 1 2 n ( n + 8 ) ) \frac { 9 }{ { 2 }^{ 12 } } \left( \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { 2 }^{ n }(n) } } \right) \quad +\quad \frac { 1 }{ { 2 }^{ 12 } } \left( \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { 2 }^{ n }(n+8) } } \right)

Now, n = 1 1 2 n ( n ) = l n 2 \sum _{ n=1 }^{ \infty }{ \frac { 1 }{ { 2 }^{ n }(n) } } = ln 2

let x = n + 8 x = n+ 8 in the 2nd series,

= 9 2 12 ( l n 2 ) + 1 2 12 ( x = 9 1 2 x 8 ( x ) ) \frac { 9 }{ { 2 }^{ 12 } } \left( ln\quad 2 \right) \quad +\quad \frac { 1 }{ { 2 }^{ 12 } } \left( \sum _{ x=9 }^{ \infty }{ \frac { 1 }{ { 2 }^{ x-8 }(x) } } \right)

= 9 2 12 ( l n 2 ) + 2 8 2 12 ( x = 9 1 2 x ( x ) ) \frac { 9 }{ { 2 }^{ 12 } } \left( ln\quad 2 \right) \quad +\quad \frac { { 2 }^{ 8 } }{ { 2 }^{ 12 } } \left( \sum _{ x=9 }^{ \infty }{ \frac { 1 }{ { 2 }^{ x }(x) } } \right)

= 9 2 12 ( l n 2 ) + 2 8 2 12 ( l n 2 n = 1 8 1 2 n ( n ) ) \frac { 9 }{ { 2 }^{ 12 } } \left( ln\quad 2 \right) \quad +\quad \frac { { 2 }^{ 8 } }{ { 2 }^{ 12 } } \left( ln\quad 2\quad -\quad \sum _{ n=1 }^{ 8 }{ \frac { 1 }{ { 2 }^{ n }(n) } } \right)

Now, computing all that, we get the value as -

0.00149 \approx \boxed{0.00149}

In response to Kartik Sharma : There is a typo of sign (+) coming from third line.

n + 9 n ( n + 8 ) = 9 8 n 1 8 ( n + 8 ) \large\frac{n+9}{n(n+8)}=\frac{9}{8n}-\frac{1}{8(n+8)}

Mas Mus - 6 years, 2 months ago
Brock Brown
Dec 28, 2014

Eight lines of Python will work just fine.

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from fractions import Fraction as frac
total = 0
i = 1
infinity = 50
while i <= infinity:
    total += frac(i+9,(2**(i+9))*i*(i+8))
    i += 1
print float(total)

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