Gif 2

Algebra Level 5

x + 2 x + 2 3 x + 3 x + 4 x + 5 x \large \lfloor x \rfloor + \lfloor 2x \rfloor + \left\lfloor \frac23 x\right\rfloor + \lfloor 3x \rfloor + \lfloor4 x \rfloor + \lfloor 5x \rfloor

If 0 x < 3 0 \le x<3 , find the total number of distinct values of the expression above.


The answer is 30.

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2 solutions

Aniket Sanghi
Sep 30, 2016

Analysed for 0 to 1

First divide the region into diff ranges to get diff integers for each . Then make these ranges into one specific range series.

Got 10 diff ranges (0,1/5)(1/5,1/4)(1/4,1/3)(1/3,2/5)(2/5,1/2)(1/2,3/5)(3/5,2/3)(2/3,3/4)(3/4,4/5)(4/5,1)

For all these the equation yields different values

By symmetry for 1 to 2 & 2 to 3 also we would get 10 diff values

Hence total is 30 values.......

i did the same but just calculated to 1/2 , it had 5 values By symmetry , the ans should be 30, by the you did u too use wolfram alpha ? @Aniket Sanghi

A Former Brilliant Member - 4 years, 8 months ago

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and it had 30 solvers uptill now ! i was the 31st , co-incidences !

A Former Brilliant Member - 4 years, 8 months ago

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Yeah! Cool coincidence! 👍

Aniket Sanghi - 4 years, 8 months ago

I didn't use wolfram alpha

Aniket Sanghi - 4 years, 8 months ago

Dammit I missed one range. I did similarly btw. Nice Sol.

Shreyash Rai - 4 years, 8 months ago

I u s e d K E I S A N A d v a n c e d C a l c u l a t o r . i n t ( 2 x / 3 ) + i n t ( x ) + i n t ( 2 x ) + i n t ( 3 x ) + i n t ( 4 x ) + i n t ( 5 x ) x 0 , . 05 , 61 F o r f ( x ) = i n t ( 2 x / 3 ) + i n t ( x ) + i n t ( 2 x ) + i n t ( 3 x ) + i n t ( 4 x ) + i n t ( 5 x ) , x f r o m 0 i n c r e a s e d b y 0.05 f o r 61 c y c l e s I g o t i n t e g e r v a l u e s f = 0 t o 41 , a n d f ( 3 ) = 47 w i t h m i s s i n g i n t e g e r s : 5 ; 11 , 12 , 13 , 14 ; 20 , 21 ; 27 , 28 , 29 , 30 ; 36. T o t a l 12. I c h e c k e d t o s e e t h e y r e a l y a r e m i s s i n g a s w e l l a s 41 i s t h e l a s t n u m b e r a s f o l l o w s . I~ used~~KEISAN~~ Advanced~Calculator.\\ \color{#3D99F6}{ int(2x/3)+int(x)+int(2x)+int(3x)+int(4x)+int(5x)}\\ x~~\color{#0f0}{ 0,.05,61}\\ For~f(x)=int(2x/3)+int(x)+int(2x)+int(3x)+int(4x)+int(5x),\\ x~from~0~increased~by~0.05~for~61~cycles~I~got\\ integer~values-~~f=0~to~41,~~and~f(3)=47\\ with~missing~integers~:-\\ 5;~\qquad~11,12,13,14;~\qquad~20,21;~\qquad~27,28,29,30;~\qquad~36.~~Total~\color{#3D99F6}{12}.\\ I~checked~to~see~they~realy~are~missing~as~well~as~41~is~the~last~number~as~follows.\\
C h e c k f o r 5 : f ( . 45 ) = 4 , f ( 5 ) = 6. f ( . 4999999999 ) a l s o = 4 5 I S m i s s i n g . C h e c k f o r 11 , 12 , 13 , 14 : f ( . 95 ) = 10 , f ( 1 ) = 15. f ( . 999999999 ) a l s o = 10 11 , 12 , 13 , 14 A R E m i s s i n g . \begin{aligned}\\ Check~for~~5~~:f(.45)&=4,~~~~~~f(5)&=6.~~\qquad~f(.4999999999)~also~&=4~~~&\implies~5~IS~missing.\\ Check~for~~11,12,13,14~~~~~:f(.95)&=10,~~~~~~f(1)&=15.~\qquad~~f(.999999999)~also~&=10~~~&\implies~11,12,13,14~ARE~missing.\\ \end{aligned}\\

S i m i l a r l y , f ( 1.45 ) = 19 , f ( 1.5 ) = 22. 20 , 21 A R E m i s s i n g . S i m i l a r l y , f ( 1.95 ) = 26 , f ( 2 ) = 31. 27 , 28 , 29 , 30 A R E m i s s i n g . S i m i l a r l y , f ( 2.45 ) = 35 , f ( 2.5 ) = 37. 36 I S m i s s i n g . \begin{aligned}\\ Similarly,~~f(1.45)&=19,~~&f(1.5)=22.~~&\implies~20,21~~ARE~missing.\\ Similarly,~~f(1.95)&=26,~~&f(2)~=31.~~&\implies~27,28,29,30~~ARE~missing.\\ Similarly,~~f(2.45)&=35,~~&f(2.5)=37.~~&\implies~36~~IS~missing.\\ \end{aligned}\\

A n d f ( 2.95 ) = 41 , f ( 3 ) = 47. f ( 2.99999999 ) a l s o = 41 , 41 I S t h e l a s t i n t e g e r . S o t o t a l 41 + 1 ( f o r 0 ) 12 = 30. N o t e t h a t , m i s s i n g n u m b e r s a r e a t i n t e r v a l s o f 0.5. F o u r e a c h b e f o r e x = 1 a n d b e f o r e x = 2. O n e e a c h b e f o r e x = 0.5 a n d b e f o r e x = 2.5. T w o b e f o r e x = 1.5. And~~~~~~f(2.95)=41,~~f(3)=47.~~~~~~~~~ f(2.99999999)~also~=41,~~\implies~ ~41~IS~the~last~integer. \\ So~total~41+1(for~0)-12=\Huge~~\color{#D61F06}{30}.\\ Note~that, ~missing~numbers~are~~at~ intervals~of~0.5.\\ Four~each~before~x=1~and~before~x=2.\\ One~each~before~x=0.5~and~before~x=2.5.\\ Two~before~x=1.5.

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