Is there a nice factorization?

256 + 128 ( 1 i 8 a i ) + 64 ( 1 i < j 8 a i a j ) + + 32 ( 1 i < j < k 8 a i a j a k ) + + a 1 a 2 a 7 a 8 = 111546435 256+128\left(\sum_{1\leq i\leq 8} a_i\right) + 64\left(\sum_{1\le i<j\le 8} a_ia_j\right) + \\ + 32\left(\sum_{1\le i<j<k\le 8} a_ia_ja_k\right) + \ldots + a_1a_2\cdots a_7a_8 = 111546435

There exist some positive integers a i a_{i} for i = 1 , 2 , . . 8 i=1,2,..8 such that they satisfy the above equation find the sum of all possible value of a 3 a_{3}


hint - 111546435 has quite interesting factors.


The answer is 82.

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1 solution

Department 8
Jan 24, 2016

Let x = 2 x=2 so the given equation becomes

x 8 + ( i = 1 8 a i ) x 7 + ( 1 i < j 8 a i a j ) x 6 + ( 1 i < j < k 8 a i a j a k ) x 5 + . . . + i = 1 8 a i = 111546435 \large{{ x }^{ 8 }+{ \left( \sum _{ i=1 }^{ 8 }{ { a }_{ i } } \right) x^{ 7 } }+\left( \sum _{ 1\le i<j\le 8 }^{ }{ { a }_{ i }{ a }_{ j } } \right) { x }^{ 6 }+\left( \sum _{ 1\le i<j<k\le 8 }^{ }{ { a }_{ i }{ a }_{ j }{ a }_{ k } } \right) { x }^{ 5 }+...+\prod _{ i=1 }^{ 8 }{ { a }_{ i } } }=111546435

Clearly this is equivalent to

i = 1 8 ( x + a i ) = 111546435 \large{\prod _{ i=1 }^{ 8 }{ \left( x+{ a }_{ i } \right) } =111546435}

Since 111546435 = 3 × 5 × 7 × 11 × 13 × 17 × 19 × 23 111546435=3\times 5\times 7\times 11\times 13\times 17\times 19\times 23 .

We assume that a 8 > a 7 > . . . . > a 1 a_{8} > a_{7} >....>a_{1} then x + a 1 = 3 a 1 = 1 x+a_{1}=3 \longrightarrow a_{1}=1 . This means we can keep changing the inequality above like a 8 > a 7 > . . . > a 1 > a 2 a_{8} > a_{7} >...>a_{1}>a_{2} this for 5 , 7 , 11 , 13 , 17 , 23 5, 7, 11, 13, 17, 23 and will get 3 , 5 , 9 , 11 , 15 , 21 3,5,9,11,15,21 respectively so the sum of all values are

1 + 3 + 5 + 9 + 11 + 15 + 21 = 82 \large{1+3+5+9+11+15+21=82} .


Things to note:

  • The sign i = 1 8 a i \large{ \sum _{ i=1 }^{ 8 }{ { a }_{ i } }} denotes a 1 + a 2 + . . . + a 8 a_{1}+a_{2}+...+a_{8}

  • The sign 1 i < j 8 a i a j \large{\sum _{ 1\le i<j\le 8 }^{ }{ { a }_{ i }{ a }_{ j } }} denotes the product of all a i a_{i} for i = 1 , 2 , . . . 8 i=1,2,...8 taken two at a time, like in the expansion of ( x + 1 ) ( x + 2 ) ( x + 3 ) (x+1)(x+2)(x+3) the coefficient of x x denotes the product of 1, 2, 3 taken to at a time like 1 × 2 + 2 × 3 + 3 × 1 1\times 2 + 2\times 3 + 3\times 1 and similarly the rest signs of he summation denotes the same thing.

  • The sign i = 1 8 a i \large{\prod _{ i=1 }^{ 8 }{ { a }_{ i } }} the denotes the product of all a i a_{i} .

Exactly! Nice work. Did you change the topic to number theory?

Shreyash Rai - 5 years, 4 months ago

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Yeah I did it.

Department 8 - 5 years, 4 months ago

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Is the same way applicable if no. is double the no. in the question? Think about it

Kaustubh Miglani - 5 years, 4 months ago

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@Kaustubh Miglani Can you please elaborate?

Department 8 - 5 years, 4 months ago

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@Department 8 Well same question just a little change,Here- 256+128(a1+a2....

................=111546435*2 There exist some positive integers a i a_{i} for i = 1 , 2 , . . 8 i=1,2,..8 such that they satisfy the above equation find the sum of all possible value of a 3 a_{3}

Kaustubh Miglani - 5 years, 4 months ago

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@Kaustubh Miglani It is pretty much tedious!

Department 8 - 5 years, 4 months ago

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@Department 8 YES it is.As expected u answered correctly congrats

Kaustubh Miglani - 5 years, 4 months ago

Too easy and a highly overrated problem

Kaustubh Miglani - 5 years, 4 months ago

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Definetely overrated. Should be lvl 3.

Shreyash Rai - 5 years, 4 months ago

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