Cubes Are King

Algebra Level 3

1 3 + 2 3 + + 2016 3 ( 1 + 2 + + 2016 ) ( 1 2 + 2 2 + + 2016 2 ) \dfrac{{1}^3+{2}^3+\cdots+{2016}^3}{({1}+{2}+\cdots+{2016})({1}^2+{2}^2+\cdots+{2016}^2)}

If the value of above expression can be expressed in the form of a b , \dfrac{a}{b}, where a a and b b are coprime positive integers, find a + b a+b .


The answer is 4036.

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2 solutions

Rishabh Jain
Feb 8, 2016

I'll rather generalise this:
Expression is of the form:
S = n 3 ( n ) ( n 2 ) \Large\mathcal{S}=\color{#007fff}{\dfrac{\displaystyle \sum n^3}{\color{#D61F06}{(\displaystyle \sum n)\color{#20A900}{(\displaystyle \sum n^2)}}}} = ( n ( n + 1 ) 2 ) 2 ( n ( n + 1 ) 2 ) ( n ( n + 1 ) ( 2 n + 1 ) 6 ) \Large =\color{#007fff}{\dfrac{(\frac{n(n+1)}{2})^{^{^2}}}{\color{#D61F06}{(\frac{n(n+1)}{2})\color{#20A900}{(\frac{n(n+1)(2n+1)}{6})}}}} = 3 2 n + 1 \Large =\dfrac{3}{2n+1} For n = 2016 , S = 3 4033 n=2016 ,~ \mathcal{S}=\dfrac{3}{4033} 4033 + 3 = 4036 \Large \therefore 4033+3=\boxed{\color{#302B94}{4036}}


Formula used : n = n ( n + 1 ) 2 n 2 = n ( n + 1 ) ( 2 n + 1 ) 6 n 3 = ( n ( n + 1 ) 2 ) 2 \small\boxed{{\color{#624F41}{\text{Formula used}}: \color{#D61F06}{\displaystyle \sum n=\dfrac{n(n+1)}{2}\\ \color{#20A900}{\displaystyle \sum n^2=\dfrac{n(n+1)(2n+1)}{6}\\ \color{#007fff}{\displaystyle \sum n^3=(\dfrac{n(n+1)}{2})^{^2}}}}}}

Cool colours and solution!

Nihar Mahajan - 5 years, 4 months ago

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T H A N K S ! \Large\color{#302B94}{\mathcal{THANKS!}}

Rishabh Jain - 5 years, 4 months ago

You mesmerized my eyes :)

Chirayu Bhardwaj - 5 years, 4 months ago

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You made my day.. I'll continue writing these type of solutions.. ;-}

Rishabh Jain - 5 years, 4 months ago

Exactly same way

Achal Jain - 5 years, 4 months ago

Thanks for the colorful solution :P

Rohit Udaiwal - 5 years, 4 months ago

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W e l c o m e \Large\mathcal{\color{#007fff}{Welcome}}

Rishabh Jain - 5 years, 4 months ago
Jun Arro Estrella
Feb 10, 2016

3 2 n + 1 \frac{3}{2n+1} is the general form Because we know that: \sum n^{3} =\(\frac{(n(n+1))^{2}}{4} ), \sum n^{2} =\(\frac{n(n+1)(2n+1)}{6} and \sum n=\(\frac{n(n+1)}{2}

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