Guess and check?

Algebra Level 4

i = 1 n ( 6 ) i = 9331 6 + 9330 \sum _{ i=1 }^{ n }{ { \left( \sqrt { 6 } \right) }^{ i } } =9331\sqrt { 6 } +9330\quad Find n n .


The answer is 11.

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1 solution

Rishabh Jain
Jul 25, 2016

Note LHS denotes the Sum of a Geometric Progression with finite terms which equals 6 ( ( 6 ) n 1 ) 6 1 = 6 ( 6 n / 2 1 ) 6 1 \dfrac{\sqrt 6((\sqrt 6)^n-1)}{\sqrt 6-1}=\dfrac{\sqrt 6( 6^{n/2}-1)}{\sqrt 6-1}

And according to question:-

6 ( 6 n / 2 1 ) 6 1 = 9331 6 + 9330 \dfrac{\sqrt 6(6^{n/2}-1)}{\sqrt 6-1}=9331\sqrt 6+9330

6 n / 2 1 = ( 9331 6 + 9330 ) ( 6 1 ) 6 \implies 6^{n/2}-1= \dfrac{(9331\sqrt 6+9330)(\sqrt 6-1)}{\sqrt 6}

6 n / 2 1 = 7776 6 1 \implies 6^{n/2}-\cancel 1=7776\sqrt 6-\cancel 1

6 n / 2 = 7776 6 = 6 5 + 1 2 = 6 11 / 2 6^{n/2}=7776\sqrt 6=6^{5+\frac 12}=6^{11/2}

n = 11 \implies \boxed{n=11}

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