Separate The Wheat

Algebra Level 5

Define the following numbers as

A = ( 1 168 + 1 175 + 1 600 ) ( 1 168 + 1 175 1 600 ) ( 1 168 + 1 600 1 175 ) ( 1 175 + 1 600 1 168 ) \mathbb{A} = \left ( \tfrac{1}{168} + \tfrac{1}{175} + \tfrac{1}{600} \right )\left ( \tfrac{1}{168} + \tfrac{1}{175} - \tfrac{1}{600} \right )\left ( \tfrac{1}{168} + \tfrac{1}{600} - \tfrac{1}{175} \right )\left ( \tfrac{1}{175} + \tfrac{1}{600} - \tfrac{1}{168} \right )

B = ( 1 240 + 1 260 + 1 624 ) ( 1 240 + 1 260 1 624 ) ( 1 240 + 1 624 1 260 ) ( 1 260 + 1 624 1 240 ) \mathbb{B} = \left ( \tfrac{1}{240} + \tfrac{1}{260} + \tfrac{1}{624} \right )\left ( \tfrac{1}{240} + \tfrac{1}{260} - \tfrac{1}{624} \right )\left ( \tfrac{1}{240} + \tfrac{1}{624} - \tfrac{1}{260} \right )\left ( \tfrac{1}{260} + \tfrac{1}{624} - \tfrac{1}{240} \right )

C = ( 1 432 + 1 540 + 1 720 ) ( 1 432 + 1 540 1 720 ) ( 1 432 + 1 720 1 540 ) ( 1 540 + 1 720 1 432 ) \mathbb{C} = \left ( \tfrac{1}{432} + \tfrac{1}{540} + \tfrac{1}{720} \right )\left ( \tfrac{1}{432} + \tfrac{1}{540} - \tfrac{1}{720} \right )\left ( \tfrac{1}{432} + \tfrac{1}{720} - \tfrac{1}{540} \right )\left ( \tfrac{1}{540} + \tfrac{1}{720} - \tfrac{1}{432} \right )

Evaluate the digit sum of 1 A + 1 B + 1 C \dfrac{1}{\sqrt{\mathbb{A}}} + \dfrac{1}{\sqrt{\mathbb{B}}} + \dfrac{1}{\sqrt{\mathbb{C}}} .


The answer is 15.

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1 solution

By Heron's Formula, we have that the area S S of the triangle with sides a , b , c a,b,c can be represented by 16 S 2 = ( a + b + c ) ( a + b c ) ( a + c b ) ( b + c a ) 16S^2 = (a+b+c)(a+b-c)(a+c-b)(b+c-a) .

For the first numbers, see that ( 1 168 ; 1 175 ; 1 600 ) = ( 25 7 24 25 ; 24 7 24 25 ; 7 7 24 25 ) \left ( \dfrac{1}{168}; \dfrac{1}{175}; \dfrac{1}{600} \right ) = \left ( \dfrac{25}{7 \cdot 24 \cdot 25}; \dfrac{24}{7 \cdot 24 \cdot 25}; \dfrac{7}{7 \cdot 24 \cdot 25} \right ) .

Because 7 2 + 2 4 2 = 2 5 2 7^2 + 24^2 = 25^2 , we have that S A = 7 24 2 ( 7 24 25 ) 2 A = 16 ( 7 24 2 ( 7 24 25 ) 2 ) 2 = 1 21000 0 2 S_{A} = \dfrac{7 \cdot 24}{2 \cdot (7 \cdot 24 \cdot 25)^2} \Leftrightarrow \mathbb{A} = 16{ \left ( \dfrac{7 \cdot 24}{2 \cdot (7 \cdot 24 \cdot 25)^2} \right ) }^2 = \dfrac{1}{210000^2} .

Similarly, for the second number we have 240 = 10 24 , 260 = 10 26 , 624 = 24 26 240 = 10 \cdot 24, \; 260 = 10 \cdot 26, \; 624 = 24 \cdot 26 and B = 1 32448 0 2 \mathbb{B} = \dfrac{1}{324480^2} . For the third number, check 432 = 18 24 , 540 = 18 30 , 720 = 24 30 432 = 18 \cdot 24, \; 540 = 18 \cdot 30, \; 720 = 24 \cdot 30 and C = 1 77760 0 2 . \mathbb{C} = \dfrac{1}{777600^2}.

Finally, 1 A = 210000 , 1 B = 324480 , 1 C = 777600 \sqrt{\dfrac{1}{\mathbb{A}}} = 210000, \; \sqrt{\dfrac{1}{\mathbb{B}}} = 324480, \; \sqrt{\dfrac{1}{\mathbb{C}}} = 777600 , so 210000 + 324480 + 777600 = 1312080 210000 + 324480 + 777600 = 1312080 . and our desired answer is 1 + 3 + 1 + 2 + 8 = 15 1+3+1+2+8 = \boxed{15} .

I think all you forgot to multiply by 16 as A turned out to be S A 2 S_{A}^{2} (and the same for B and C) meaning that the final answer was 4 times of the actual. However the answer still remained :)

Joel Tan - 6 years, 6 months ago

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As planned, it all cuts out (lovely ratios).

Guilherme Dela Corte - 6 years, 6 months ago

I got A = 1 2756250000 , B = 1 6580454400 , C = 1 37791360000 A=\frac{1}{2756250000}, B=\frac{1}{6580454400}, C=\frac{1}{37791360000}

which is equivalent to A = 1 5250 0 2 , B = 1 8112 0 2 , C = 1 19440 0 2 A=\frac{1}{52500^2}, B=\frac{1}{81120^2}, C=\frac{1}{194400^2}

and 52500 + 81120 + 194400 = 328020 52500+81120+194400=328020

the final answer to enter 3 + 2 + 8 + 0 + 2 + 0 = 15 3+2+8+0+2+0=\boxed{15}

Bob Kadylo - 4 years, 1 month ago

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