The pillar of Infinity

Algebra Level 4

The fraction 1 2 \frac{1}{2} grows every second in the following manner:

1 2 1 4 2 8 1 4 4 16 2 8 8 32 \large \frac{1}{2} \to \large{ \frac{\frac{1}{4}}{\frac{2}{8}}} \to \huge{\frac{\frac{\frac{1}{4}}{\frac{4}{16}}}{\frac{\frac{2}{8}}{\frac{8}{32}}}}

Given that the expression at time t = 0 t=0 is 1 2 \frac12 and grows every second to the next expression in the same pattern as above. Find the sum of all the expressions formed till 3600 3600 seconds. ​


This question is part of the set All-Zebra


The answer is 3600.5.

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2 solutions

Ashish Menon
Apr 26, 2016

Answer at t = 1 is 1.
Answer at t = 2 is 1.
Answer at t = 3 is 1.
Answer at t = 4 is 1.


If we look carefully all the patterns are of the form x y z x z y \dfrac{\dfrac{x}{y}}{\dfrac{zx}{zy}} , which is 1, so the answer is 0.5 + (1 × 3600) = 3600.5 \boxed{3600.5}

Shawn Franchi
Apr 27, 2016

An alternate solution is given roughly by 1.27447 × 10 11241 1.27447 \times {10}^{11241} . We note that

1 2 = 2 ( 1 1 ) ! 3 ( 1 ) 1 \huge \frac { 1 }{ 2 } =\frac { 2(1-1)! }{ 3-{(-1)}^{1}}

1 / 4 2 / 8 = 2 ( 2 1 ) ! 3 ( 1 ) 2 = 1 \huge \frac { { 1 }/{ 4 } }{ { 2 }/{ 8 } } =\frac { 2(2-1)! }{ 3-(-1)^{ 2 } } =\quad 1

1 / 4 4 / 16 2 / 8 8 / 32 = 2 ( 3 1 ) ! 3 ( 1 ) 3 = 1 \huge \frac { \frac { 1 / 4 }{ 4/16 } }{ \frac { 2/8 }{ 8/32 } } =\frac { 2(3-1)! }{ 3-(-1)^{ 3 } } = 1 .

Continuing this pattern, we may let the n n th term of the sequence be given by

2 ( n 1 ) ! 3 ( 1 ) n \huge \frac { 2(n-1)! }{ 3-{ (-1) }^{ n } } .

Thus the series is given by

n = 1 3601 2 ( n 1 ) ! 3 ( 1 ) n 1.2744 × 10 11241 \huge \sum _{ n=1 }^{ 3601 }{\quad \frac { 2(n-1)! }{ 3-{ (-1) }^{ n } } } \cong 1.2744\times { 10 }^{ 11241 } .

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