Given that 1 0 0 2 0 0 4 0 0 8 0 1 6 0 3 2 has a prime factor " P " which is greater than 2 5 0 0 0 0 , find P
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Great intuition!!!!!
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but how your brain goes in that direction???please tell i want also to be a genius like you @Karthik Sharma
S p l i t i t u p a n d w r i t e n e a t l y t o g e t :
1 0 0 2 0 0 4 0 0 8 0 1 6 0 3 2 = 2 0 ∗ 1 0 1 5 + 2 1 ∗ 1 0 1 2 + 2 2 ∗ 1 0 9 + 2 3 ∗ 1 0 6 + 2 4 ∗ 1 0 3 + 2 5
A l s o n o t i c e t h a t
( x − y ) ( x 6 − y 6 ) = ( x + y ) ( x 2 + x y + y 2 ) ( x 2 − x y + y 2 )
= x 5 + y x 4 + y 2 x 3 + y 3 x 2 + y 4 x + y 5
N o w l e t x = 1 0 3 a n d y = 2 S o ,
1 0 0 2 0 0 4 0 0 8 0 1 6 0 3 2 = 1 0 0 2 ∗ 9 9 8 0 0 4 ∗ 1 0 0 2 0 0 4 = 1 0 0 2 ∗ 4 ∗ 2 4 9 5 0 1 ∗ 4 ∗ 2 5 0 5 0 1
A s b y t h e q u e s t i o n t h e r e i s a p r i m e P > 2 5 0 0 0 0 .
S o w i t h o u t f u r t h e r p r o v i n g t h a t 2 5 0 5 0 1 i s a p r i m e ,
w e c a n a s s u m e t h a t P = 2 5 0 5 0 1
It is an extremely easy problem. Took only a few seconds to solve. It should have been a level 2 problem.
Combining elements of both Karthik Sharma 's and Joseph Varghese 's solutions:
We first note that 3 , 7 divides our number, which will be important later.
We now consider making use of factorisation below:
1 0 0 0 5 + 1 0 0 0 4 ∗ 2 1 + ⋯ + 1 0 0 0 1 ∗ 2 4 + 2 5 = 1 0 0 0 − 2 1 0 0 0 6 − 2 6
= ( 1 0 0 0 + 2 ) ( 1 0 0 0 2 + 1 0 0 0 ∗ 2 + 2 2 ) ( 1 0 0 0 2 − 1 0 0 0 ∗ 2 + 2 2 )
= 1 0 0 2 ∗ 9 9 8 0 0 4 ∗ 1 0 0 2 0 0 4
And we note that 1 0 0 2 = 2 ∗ 3 ∗ 1 6 7 , 9 9 8 0 0 4 = 4 ∗ 3 ∗ 7 ∗ 1 1 8 8 1 , since as we've observed since 3 , 7 do not divide 1 0 0 2 0 0 4 , they must divide some of the other factors.
1 0 0 2 0 0 4 = 4 ∗ 2 5 0 5 0 1 , which so far is the only non-factored factor of out number that we have so far that is > 2 5 0 0 0 0
(Side note: 1 1 8 8 1 further factorises to 1 0 9 2 , but that's unimportant as it is already < 2 5 0 0 0 0 )
Hence, our desired factor is thus 2 5 0 5 0 1
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By multiplying the 1st 7 digits by 8 we get the last 9 digits as result.
⇒ 1 0 0 2 0 0 4 is a factor of the 1 0 0 2 0 0 4 0 0 8 0 1 6 0 3 2
And, 1 0 0 2 0 0 4 = 2 × 2 × 2 5 0 5 0 1
Given that P > 2 5 0 0 0 0 , And P is Prime,
2 5 0 5 0 1 is the answer.