Inspired by Pi Han

0.22222 4 0.33333 5 0.22222 \ldots _ 4 \\ 0.33333 \ldots _5

Which is larger?


Inspiration

0.33333 5 0.33333 \ldots _5 They are equal 0.22222 4 0.22222 \ldots _4

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1 solution

Zee Ell
Oct 22, 2016

0.222 2 ˙ 4 = 2 4 1 + 2 4 2 + 2 4 3 + . . . = 2 ( 1 4 1 + 1 4 2 + 1 4 3 + . . . ) = 0.222 \dot {2} _4 = \frac {2}{4^1} + \frac {2}{4^2} + \frac {2}{4^3} + ... = 2( \frac {1}{4^1} + \frac {1}{4^2} + \frac {1}{4^3} + ... ) =

= 2 ( 1 4 1 1 4 ) = 2 ( 1 4 3 4 ) = 2 × 1 3 = 2 3 10 = 2 ( \frac { \frac {1}{4} }{ 1 - \frac {1}{4} } ) = 2 ( \frac { \frac {1}{4} }{ \frac {3}{4} } ) = 2 × \frac {1}{3} = \frac {2}{3} _ {10}

0.333 3 ˙ 5 = 3 5 1 + 3 5 2 + 3 5 3 + . . . = 3 ( 1 5 1 + 1 5 2 + 1 5 3 + . . . ) = 0.333 \dot {3} _5 = \frac {3}{5^1} + \frac {3}{5^2} + \frac {3}{5^3} + ... = 3( \frac {1}{5^1} + \frac {1}{5^2} + \frac {1}{5^3} + ... ) =

= 3 ( 1 5 1 1 5 ) = 3 ( 1 5 4 5 ) = 3 × 1 4 = 3 4 = 3 ( \frac { \frac {1}{5} }{ 1 - \frac {1}{5} } ) = 3 ( \frac { \frac {1}{5} }{ \frac {4}{5} } ) = 3 × \frac {1}{4} = \frac {3}{4}

Since 3 4 > 2 3 9 12 > 8 12 , therefore our answer is: \text {Since } \frac {3}{4} > \frac {2}{3} \iff \frac {9}{12} > \frac {8}{12} \text { , therefore our answer is:}

0.33333.. . 5 \boxed {0.33333..._5}

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