Series 4

Algebra Level 4

{ i = 1 i 2 2 i = m i = 1 m 2 i i = 1 m 2 i 3 = R Z \begin{cases} \displaystyle \sum_{i=1}^{\infty}\frac{i^{2}}{2^{i}}=m \\ \displaystyle\frac{\displaystyle \sum_{i = 1}^{m^2}i}{\displaystyle \sum_{i = 1}^{m^2}i^3} = \frac{R}{Z} \end{cases}

Here R R and Z Z are positive co-prime integers.

Find m 2 + R + Z m^{2}+R+Z .


The answer is 703.

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1 solution

Mateus Gomes
Jan 22, 2016

i = 1 i 2 2 i = m \displaystyle \sum_{i=1}^{\infty}\frac{i^{2}}{2^{i}}=m i = 1 i 2 2 i = S 1 = 1 2 2 1 + 2 2 2 2 + 3 2 2 3 + 4 2 2 4 . . . ( 1 ) \sum_{i=1}^{\infty}\frac{i^{2}}{2^{i}}=S_1=\frac{1^{2}}{2^{1}}+\frac{2^{2}}{2^{2}}+\frac{3^{2}}{2^{3}}+\frac{4^{2}}{2^{4}}...~~(1) S 1 2 = 1 2 2 2 + 2 2 2 3 + 3 2 2 4 + 4 2 2 5 . . . ( 2 ) ~~~~~~~~~~~\frac{{S_1}}{2}=\frac{1^{2}}{2^{2}}+\frac{2^{2}}{2^{3}}+\frac{3^{2}}{2^{4}}+\frac{4^{2}}{2^{5}}...~~(2) S 1 2 = 1 2 1 + 3 2 2 + 5 2 3 + 7 2 4 . . . ( 1 ) ( 2 ) ~~~~~~~~~~~~~~~~~\rightarrow\frac{S_1}{2}=\frac{1}{2^{1}}+\frac{3}{2^{2}}+\frac{5}{2^{3}}+\frac{7}{2^{4}}...~~(1)-(2) S 1 4 = 1 2 2 + 3 2 3 + 5 2 4 + 7 2 5 . . . ( 3 ) ~~~~~~~\rightarrow\frac{S_1}{4}=\frac{1}{2^{2}}+\frac{3}{2^{3}}+\frac{5}{2^{4}}+\frac{7}{2^{5}}...~~(3) S 1 4 = 1 2 1 + 2 ( 1 2 2 + 1 2 3 + 1 2 4 . . . ) ( 1 ) ( 2 ) ( 3 ) ~~~~~\rightarrow\frac{S_1}{4}=\frac{1}{2^{1}}+2(\frac{1}{2^{2}}+\frac{1}{2^{3}}+\frac{1}{2^{4}}...)~~(1)-(2)-(3) S 1 4 = 1 2 1 + 1 ~~~~~\rightarrow\frac{S_1}{4}=\frac{1}{2^{1}}+1 S 1 = 6 \rightarrow\color{#3D99F6}{\boxed{S_1=6}} i = 1 36 i i = 1 36 i 3 = ( 36 ) ( 37 ) 2 ( 3 6 2 ) ( 3 7 2 ) 4 = 1 ( 18 ) ( 37 ) = R Z \displaystyle\frac{\displaystyle \sum_{i = 1}^{36}i}{\displaystyle \sum_{i = 1}^{36}i^3}=\frac{\frac{(36)(37)}{2}}{\frac{(36^2)(37^2)}{4}}=\frac{1}{(18)(37)}=\frac{R}{Z} m 2 + R + Z = 36 + 1 + ( 18 ) ( 37 ) = 703 \Large\rightarrow\color{#3D99F6}{\boxed{m^{2}+R+Z=36+1+(18)(37)=703}}

I don't get the third step where you subtracted the two equations .. can u explain

Dhruv Aggarwal - 5 years, 4 months ago

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third step is a AGP( https://brilliant.org/wiki/arithmetic-geometric-progression/)

Mateus Gomes - 5 years, 4 months ago

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Okay ya Thanks ..

Dhruv Aggarwal - 5 years, 4 months ago

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