Repunits!!

A repunit is a number that contains only the digit 1. If 111 111 = 2222 2222 + ( 333 333 ) 2 111\ldots 111= 2222\ldots 2222 + (333\ldots 333)^2 With 1 occurring x x times and 2 and 3 occurring y y times then what is x / y x/y ?


The answer is 2.

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4 solutions

I solved this way:

Fist, let rewrite the second member as follows:

= 2 × 111...111 y t i m e s + 9 × ( 111...111 y t i m e s ) 2 =2\times \underbrace { 111...111 }_{ y\quad times } +9\times ({ \underbrace { 111...111 }_{ y\quad times } })^{ 2 }

Extracting the common factor and multiplying again we have:

= 111...111 y t i m e s × ( 2 + 999...999 y t i m e s ) =\underbrace { 111...111 }_{ y\quad times } \times ({ 2+\underbrace {999...999 }_{ y\quad times } })

But we have that 999...999 y t i m e s = 10 y 1 { \underbrace { 999...999 }_{ y\quad times } }={ 10 }^{ y }-1

Therefore, the second member becomes:

= 111...111 y t i m e s × ( 10 y + 1 ) =\underbrace { 111...111 }_{ y\quad times } \times ({ { { 10 }^{ y }+1 } })

Applying the distributive propriety, we have that

= 111...111 y t i m e s × 10 y + 111...111 y t i m e s =\underbrace { 111...111 }_{ y\quad times } \times { 10 }^{ y }+\underbrace { 111...111 }_{ y\quad times }

= 111...111 y t i m e s 000...000 y t i m e s 2 y + 111...111 y t i m e s =\underbrace { \underbrace { 111...111 }_{ y\quad times } \underbrace { 000...000 }_{ y\quad times } }_{ 2y } +\underbrace { 111...111 }_{ y\quad times }

Wich is equal to:

= 111...111 2 y =\underbrace { 111...111 }_{ 2y }

Hence,

111...111 x = 111...111 2 y \underbrace { 111...111 }_{ x } =\underbrace { 111...111 }_{ 2y }

Finally, we must have

x = 2 × y x=2\times{y} ,

Therefore, x y = 2 \frac { x }{ y } =\boxed{2}

Q . E . D . \quad \quad \quad Q.E.D.

A good solution.

Malay Pandey - 7 years, 2 months ago

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Thank you!

Carlos E. C. do Nascimento - 7 years, 2 months ago

Starting from a smaller pattern:

11 = 2 + 3 2 , 11=2+3^{2}, where x = 2 , y = 1 x=2, y=1

1111 = 22 + 3 3 2 , 1111=22+33^{2}, where x = 4 , y = 2 x=4, y=2

111111 = 222 + 33 3 2 , 111111=222+333^{2}, where x = 6 , y = 3 x=6, y=3

This will continue forever with the pattern of y = 1 2 x y=\frac{1}{2}x

So, the answer must be x y = 2 \frac{x}{y}=\boxed{2}

But you must also give a genuine solution to the problem

Malay Pandey - 7 years, 2 months ago

This is a perfectly valid trollution.

Daniel Wang - 7 years, 2 months ago

I did the same way.....CHEERS!!!

Vighnesh Raut - 7 years, 2 months ago

even i did it this way

Kishore Ravisankar - 7 years, 2 months ago

i madde use of G.P. 111....111(x times) = 1 + 10 + 100+ 1000 ...... and so on with finally x terms and common ratio 10 so 111....111(x times) = ((10 ^ x) - 1) / 9 thus L.H.S. = (( 10 ^ x ) - 1)/9 R.H.S. = 2*( ( 10 ^ y - 1) / 9 ) + 9 * { ( 10 ^ y - 1) / 9 } ^ 2 and solve

Amogh Jain
Apr 1, 2014

Simple approach is 2+3^2 is 11 so x=2 and y=1. Therefore x/y=2.

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