Sum this 2015

Algebra Level 4

n = 1 2015 16 n 48 ( n + 6 ) ( n + 3 ) ( n ) = ? \large{\sum _{ n=1 }^{ 2015 }{ \dfrac { 16n-48 }{ ( n+6 ) ( n+3 ) ( n ) } }=\ ?}

Give your answer upto 3 decimal places.


The answer is 0.03648.

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1 solution

n = 1 2015 16 n 48 ( n + 6 ) ( n + 3 ) ( n ) = n = 1 2015 ( 16 ( n + 3 ) ( n + 6 ) 48 n ( n + 3 ) ( n + 6 ) ) = n = 1 2015 ( 16 3 [ 1 n + 3 1 n + 6 ] 48 18 [ 1 n 2 n + 3 + 1 n + 6 ] ) = 8 3 n = 1 2015 ( 1 n + 4 n + 3 3 n + 6 ) = 8 3 ( n = 1 2015 1 n + n = 1 2015 1 n + 3 + 3 n = 1 2015 1 n + 3 3 n = 1 2015 1 n + 6 ) = 8 3 ( n = 1 2015 1 n + n = 4 2018 1 n + 3 [ n = 4 2018 1 n n = 7 2021 1 n ] ) = 8 3 ( 1 1 1 2 1 3 + 1 2016 + 1 2017 + 1 2018 + 3 [ 1 4 + 1 5 + 1 6 1 2019 1 2020 1 2021 ] ) = 0.037 \begin{aligned} \sum_{n=1}^{2015} \frac{16n-48}{(n+6)(n+3)(n)} & = \sum_{n=1}^{2015} \left( \frac{16}{(n+3)(n+6)} - \frac{48}{n(n+3)(n+6)} \right) \\ & = \sum_{n=1}^{2015} \left( \frac{16}{3} \left[ \frac{1}{n+3} - \frac{1}{n+6} \right] - \frac{48}{18} \left[ \frac{1}{n} - \frac{2}{n+3} + \frac{1}{n+6} \right] \right) \\ & = \frac{8}{3} \sum_{n=1}^{2015} \left( - \frac{1}{n} + \frac{4}{n+3} - \frac{3}{n+6} \right) \\ & = \frac{8}{3} \left(- \sum_{n=1}^{2015} \frac{1}{n} + \sum_{n=1}^{2015} \frac{1}{n+3} + 3 \sum_{n=1}^{2015} \frac{1}{n+3}- 3 \sum_{n=1}^{2015} \frac{1}{n+6} \right) \\ & = \frac{8}{3} \left(- \sum_{n=1}^{2015} \frac{1}{n} + \sum_{n=4}^{2018} \frac{1}{n} + 3 \left[\sum_{n=4}^{2018} \frac{1}{n} - \sum_{n=7}^{2021} \frac{1}{n} \right] \right) \\ & = \frac{8}{3} \left( - \frac{1}{1} - \frac{1}{2} - \frac{1}{3} + \frac{1}{2016} + \frac{1}{2017} + \frac{1}{2018} + 3\left[\frac{1}{4} + \frac{1}{5} + \frac{1}{6} - \frac{1}{2019} - \frac{1}{2020} - \frac{1}{2021} \right] \right) \\ & = \boxed{0.037} \end{aligned}

I am unable to understand what you did in last step

Atul Shivam - 5 years, 8 months ago

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I have added details to my solution. See if you can understand now.

Chew-Seong Cheong - 5 years, 8 months ago

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Thank you now it is clear to me

Atul Shivam - 5 years, 8 months ago

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