Maximum of e

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Let a , b , c , d , e a, b, c, d, e be positive integers which satisfies a < b < c < d < e a < b < c < d < e and 1 a + 1 b + 1 c + 1 d + 1 e = 1 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} + \frac{1}{e} = 1 What are the last three digits of the maximum value of e e ?


The answer is 806.

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1 solution

Piyushkumar Palan
Dec 31, 2013

To maximize e e , lets minimize a , b , c , d . a, b, c, d.

Clearly, none of them can be 1 1 .

Lets Set a = 2 a = 2 & get smallest possible integers: b , c , d b, c, d in increasing order satisfying given equation.

1 2 + 1 b + 1 c + 1 d + 1 e = 1 1 b + ( 1 c + 1 d + 1 e ) = 1 2 \frac{1}{2} + \frac{1}{b} + \frac{1}{c} +\frac{1}{d} +\frac{1}{e} = 1 \implies \frac{1}{b} + (\frac{1}{c} +\frac{1}{d} +\frac{1}{e}) = \frac{1}{2}

Let ( 1 c + 1 d + 1 e ) = 1 x 1 b + 1 x = 1 2 (\frac{1}{c} +\frac{1}{d} +\frac{1}{e} ) = \frac{1}{x} \implies \frac{1}{b} + \frac{1}{x} = \frac{1}{2}

Rearranging, 2 ( b + x ) = b x ( b 2 ) ( x 2 ) = 4 = 1 × 4. 2(b + x) = bx \implies (b - 2)(x - 2) = 4 = 1 \times 4.

Set ( b 2 ) = 1 , ( x 2 ) = 4 (b - 2) = 1, (x - 2) = 4 to minimize b. b = 3 , x = 6 ( 1 c + 1 d + 1 e ) = 1 6 \implies b = 3, x = 6 \implies (\frac{1}{c} +\frac{1}{d} +\frac{1}{e} ) = \frac{1}{6}

Repeating same procedure, let ( 1 d + 1 e ) = 1 y (\frac{1}{d} +\frac{1}{e} ) = \frac{1}{y} ( 1 c + 1 y ) = 1 6 ( c 6 ) ( y 6 ) = 36 = 1 × 36 ( \frac{1}{c} +\frac{1}{y} ) = \frac{1}{6} \implies (c - 6)(y-6) = 36 = 1 \times 36

Set c = 7 , y = 42 1 d + 1 e = 1 42 ( d 42 ) ( e 42 ) = 1 × 1764 c = 7, y = 42 \implies \frac{1}{d} +\frac{1}{e} = \frac{1}{42} \implies (d-42)(e-42) = 1\times 1764

Set d = 43 , e = 1806 d = 43, e = 1806 \implies answer 806 \boxed{806}

You would have to be careful with substantiating the first statement. To maximize e e , we want to minimize 1 e \frac{1}{e} which means we want to maximize 1 a + 1 b + 1 c + 1 d \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} . This doesn't immediately imply that we want to minimize each of a a and b b and c c and d 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 a , we can get a smaller value of b / c / d b/c/d which leads to a much smaller value of 1 e \frac{1}{e} , hence maximizing the value of e e .

Calvin Lin Staff - 7 years, 5 months ago

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I suspected that, but didn't work out enough. Thanks for clarification.

Piyushkumar Palan - 7 years, 5 months ago

It's so good

RAGHU RAM - 7 years, 5 months ago

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