Let a , b , c , d , e be positive integers which satisfies a < b < c < d < e and a 1 + b 1 + c 1 + d 1 + e 1 = 1 What are the last three digits of the maximum value of e ?
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You would have to be careful with substantiating the first statement. To maximize e , we want to minimize e 1 which means we want to maximize a 1 + b 1 + c 1 + d 1 . This doesn't immediately imply that we want to minimize each of a and b and c and d .
The greedy algorithm doesn't always work (especially for Egyptian fractions), and we may not always get a solution. Furthermore, it is possible that by using a larger value for a , we can get a smaller value of b / c / d which leads to a much smaller value of e 1 , hence maximizing the value of e .
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I suspected that, but didn't work out enough. Thanks for clarification.
It's so good
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To maximize e , lets minimize a , b , c , d .
Clearly, none of them can be 1 .
Lets Set a = 2 & get smallest possible integers: b , c , d in increasing order satisfying given equation.
2 1 + b 1 + c 1 + d 1 + e 1 = 1 ⟹ b 1 + ( c 1 + d 1 + e 1 ) = 2 1
Let ( c 1 + d 1 + e 1 ) = x 1 ⟹ b 1 + x 1 = 2 1
Rearranging, 2 ( b + x ) = b x ⟹ ( b − 2 ) ( x − 2 ) = 4 = 1 × 4 .
Set ( b − 2 ) = 1 , ( x − 2 ) = 4 to minimize b. ⟹ b = 3 , x = 6 ⟹ ( c 1 + d 1 + e 1 ) = 6 1
Repeating same procedure, let ( d 1 + e 1 ) = y 1 ( c 1 + y 1 ) = 6 1 ⟹ ( c − 6 ) ( y − 6 ) = 3 6 = 1 × 3 6
Set c = 7 , y = 4 2 ⟹ d 1 + e 1 = 4 2 1 ⟹ ( d − 4 2 ) ( e − 4 2 ) = 1 × 1 7 6 4
Set d = 4 3 , e = 1 8 0 6 ⟹ answer 8 0 6