Given the Diophantine equation :
a 2 + b 2 + c 2 = 2 0 4 1
How many ordered integer ( a , b , c ) solutions exist?
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Alright. But can you use mathematical approach to solve this problem?
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Nope. I can assume a is even and start with a = 0 , 2 , 4 , ⋯ 2 2 . It is going to be very long.
By using logic and brute force, we have 9 trios: ( 4 5 , 4 , 0 ) , ( 4 2 , 1 4 , 9 ) , ( 4 1 , 1 8 , 6 ) , ( 4 0 , 2 1 , 0 ) , ( 3 9 , 2 2 , 6 ) , ( 3 9 , 1 8 , 1 4 ) , ( 3 6 , 2 7 , 4 ) , ( 3 6 , 2 4 , 1 3 ) , ( 3 2 , 2 4 , 2 1 ) .
Now it's time for combinatorics.
Since the problem asked for the integer solutions, we have to include both positive- and negative-containing solutions. Of the given result, we see that there are 7 all-nonzero trios, which can be shuffled (For example: ( 4 5 , 4 , 0 ) and ( 4 , 4 5 , 0 ) or ( 0 , 4 , 4 5 ) are different) in 3 ! ways and have 2 3 combinations of positive or negative value for each trios.
For the remaining two, we also have 3 ! ways to shuffle but only 2 2 positive-or-negative-value combinations since 0 is a neutral number (it's neither positive nor negative).
The answer is 7 × 3 ! × 2 3 + 2 × 3 ! × 2 2 = 3 8 4 different solutions.
You should have put the option wisely . The answer will always divisible by 6 and 8Anyone can just check the divide the option by 6 and guess the answer
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You cannot know if there are solutions with the same number occuring multiple times in the triplet without solving the equation in some form.
Well, I don't think so. Guessing these kinds of problem is difficult because they involve in combinatorics.
Also I gave 3 on 8 answer that are divisible by 24 so that the probability of guessing the correct one is 3 1 , which make it more difficult (Unless you use online website like Wolfram|Alpha).
Anh Khoa, I have changed it to the proper word "ordered" .
but how could we find the 9 sets? just by brute force? could we find out a more beautiful way
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As all I know, there are no other method for these kinds than using logic and brute force.
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Since brute force is to be used, we must well have computer doing the work. Using the Code environment of Brilliant.org: