I am boxing sequences

Algebra Level 5

Let { x n } \{ x _n \} be a sequence defined such that x k + 1 = x k 2 + x k { x }_{ k+1 }={ { x_{ k } }^{ 2 } } +{ x }_{ k } with x 1 = 1 2 x_1 = \frac 1 2 .

Find the greatest integer less than or equals to the expression below.

1 x 1 + 1 + 1 x 2 + 1 + + 1 x 100 + 1 \large \frac { 1 }{ { x }_{ 1 }+1 } +\frac { 1 }{ { x }_{ 2 }+1 } + \ldots + \frac { 1 }{ { x }_{ 100 }+1 }


The answer is 1.

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1 solution

Karan Shekhawat
Apr 23, 2015

S = k = 1 100 1 x k + 1 = k = 1 100 x k x k ( x k + 1 ) { x k ( x k + 1 ) = x k + 1 x k 2 = x k + 1 x k S = k = 1 100 x k x k + 1 = k = 1 100 x k 2 ( x k + 1 ) x k S = k = 1 100 x k + 1 x k ( x k + 1 ) x k = k = 1 100 1 x k 1 x k + 1 { T e l e s c o p i c } S = 1 x 1 1 x 100 = 2 1 x 100 \displaystyle{S=\sum _{ k=1 }^{ 100 }{ \cfrac { 1 }{ { x }_{ k }+1 } } =\sum _{ k=1 }^{ 100 }{ \cfrac { { x }_{ k } }{ { x }_{ k }({ x }_{ k }+1) } } \\ \because \begin{cases} { x }_{ k }({ x }_{ k }+1)={ x }_{ k+1 } \\ { { x }_{ k } }^{ 2 }={ x }_{ k+1 }-{ x }_{ k } \end{cases}\\ S=\sum _{ k=1 }^{ 100 }{ \cfrac { { x }_{ k } }{ { x }_{ k+1 } } } =\sum _{ k=1 }^{ 100 }{ \cfrac { { { x }_{ k } }^{ 2 } }{ { (x }_{ k+1 }){ x }_{ k } } } \\ S=\sum _{ k=1 }^{ 100 }{ \cfrac { { x }_{ k+1 }-{ x }_{ k } }{ { (x }_{ k+1 }){ x }_{ k } } } =\sum _{ k=1 }^{ 100 }{ \cfrac { 1 }{ { x }_{ k } } -\cfrac { 1 }{ { x }_{ k+1 } } } \\ \left\{ \because Telescopic \right\} \\ S=\cfrac { 1 }{ { x }_{ 1 } } -\cfrac { 1 }{ { x }_{ 100 } } =2-\cfrac { 1 }{ { x }_{ 100 } } \\ }

Now we don't need to evaluate x 100 { x }_{ 100 } But we know that given sequence is Increasing , Since It's Nature is Parabolic, So it's increses rapidly , and clearly , x 3 > 1 x 100 > > > 1 1 x 100 < < < 1 1 < S < 2 S = 1 \displaystyle{{ x }_{ 3 }>1\Rightarrow { x }_{ 100 }>>>1\Rightarrow \cfrac { 1 }{ { x }_{ 100 } } <<<1\\ 1^{ - }<S<{ 2 }^{ - }\\ \boxed { \left\lfloor S \right\rfloor =1 } }

Yes.. same , Nice Question ! Loved to working on it!

Nishu sharma - 6 years, 1 month ago

nice one... i too did the same thing..

Sriram Vudayagiri - 6 years ago

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