A question of computer science?

Algebra Level 5

{ ( 3 + 8 ) 101 } + ( 3 8 ) 101 = ? \large \left \{(3+\sqrt{8})^{101} \right \} + (3-\sqrt{8})^{101} = \, ?

Give your answer to 3 decimal places.

Notation : { } \{ \cdot \} denotes the fractional part function .


The answer is 1.000.

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1 solution

Mark Hennings
Sep 11, 2016

By the Binomial Theorem, ( 3 + 8 ) 101 + ( 3 8 ) 101 = 2 n = 0 50 ( 101 2 n ) × 3 101 2 n × 8 n (3+\sqrt{8})^{101} + (3-\sqrt{8})^{101} = 2\sum_{n=0}^{50}\binom{101}{2n}\times 3^{101-2n}\times8^n Is a positive integer, and so { ( 3 + 8 ) 101 } + ( 3 8 ) 101 \big\{(3+\sqrt{8})^{101}\big\} + (3-\sqrt{8})^{101} Is also a positive integer. But 3 8 = ( 3 + 8 ) 1 3-\sqrt{8} = (3+\sqrt{8})^{-1} , and so both { ( 3 + 8 ) 101 } \big\{(3+\sqrt{8})^{101}\big\} and ( 3 8 ) 101 (3-\sqrt{8})^{101} lie strictly between 0 0 and 1 1 . Since their sum is an integer, that sum must be 1 \boxed{1} .

I used demoivres theorem to get the same answer, typed it in wrong of course.

Joe Potillor - 4 years, 7 months ago

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