Sophie would have solved it easily!

Level 2

Simplify ( 3 4 + 4 ) ( 7 4 + 4 ) ( 9 9 4 + 4 ) ( 1 4 + 4 ) ( 5 4 + 4 ) ( 9 7 4 + 4 ) \dfrac{(3^4+4)(7^4+4)\ldots(99^4+4)}{(1^4+4)(5^4+4)\ldots(97^4+4)}


The answer is 10001.

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1 solution

Gabriel Chacón
Apr 22, 2019

By Sophie Germain identity, we know that a 4 + 4 b 4 = ( a 2 + 2 b 2 2 a b ) ( a 2 + 2 b 2 + 2 a b ) = ( a ( a 2 b ) + 2 b 2 ) ( a ( a + 2 b ) + 2 b 2 ) a^4+4b^4=(a^2+2b^2-2ab)(a^2+2b^2+2ab)=(a(a-2b)+2b^2)(a(a+2b)+2b^2) .

All factors are of the form x 4 + 4 x^4+4 , so using the identity with a = 1 , 3 , 5 , 7 , a=1,3,5,7,\ldots and b = 1 b=1 we get:

[ ( 3 ( 3 2 ) + 2 ) ( 3 ( 3 + 2 ) + 2 ) ] [ ( 7 ( 7 2 ) + 2 ) ( 7 ( 7 + 2 ) + 2 ) ] [ ( 99 ( 99 2 ) + 2 ) ( 99 ( 99 + 2 ) + 2 ) ] [ ( 1 ( 1 2 ) + 2 ) ( 1 ( 1 + 2 ) + 2 ) ] [ ( 5 ( 5 2 ) + 2 ) ( 5 ( 5 + 2 ) + 2 ) ] [ ( 97 ( 97 2 ) + 2 ) ( 97 ( 97 + 2 ) + 2 ) ] \dfrac{[\textcolor{#D61F06}{(3(3-2)+2)}\textcolor{#3D99F6}{(3(3+2)+2)}][\textcolor{#20A900}{(7(7-2)+2)}\textcolor{#BBBBBB}{(7(7+2)+2)}]\ldots[\textcolor{#D61F06}{(99(99-2)+2)}(99(99+2)+2)]}{[(1(1-2)+2)\textcolor{#D61F06}{(1(1+2)+2)}][\textcolor{#3D99F6}{(5(5-2)+2)}\textcolor{#20A900}{(5(5+2)+2)}]\ldots[\textcolor{#BBBBBB}{(97(97-2)+2)}\textcolor{#D61F06}{(97(97+2)+2)}]}

All factors cancel except the first in the denominator and the last in the numerator yielding a value for the fraction of 10001 \boxed{10001}

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