TRUE or FALSE ?
There exist(s) a triplet/triplets of positive integers ( a , b , c ) such that ( a + b + c ) 3 = a 3 + b 3 + c 3 .
Or Geometrically speaking,
There exist three cubes with integer side lengths a , b and c , respectively, such that the sum of their volumes equals the volume of another cube with side length ( a + b + c ) .
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For 1st question, a=b=c=0 is possible.
( a + b + c ) 3 = a 3 + b 3 + c 3 a 3 + b 3 + c 3 + 3 a 2 b + 3 a 2 c + 3 a b 2 + 3 b 2 c + 3 a c 2 + 3 b c 2 + 6 a b c = a 3 + b 3 + c 3 a 2 b + a 2 c + a b 2 + b 2 c + a c 2 + b c 2 + 2 a b c = 0 a 2 c + 2 a b c + b 2 c + a 2 b + a b 2 + a c 2 + b c 2 = 0 c ( a 2 + 2 a b + b 2 ) + a b ( a + b ) + c 2 ( a + b ) = 0 c ( a + b ) 2 + ( a b + c 2 ) ( a + b ) = 0 ( c ( a + b ) + a b + c 2 ) ( a + b ) = 0 ( a b + a c + b c + c 2 ) ( a + b ) = 0 ( a ( b + c ) + c ( b + c ) ) ( a + b ) = 0 ( a + c ) ( b + c ) ( a + b ) = 0 Therefore either a = − b , a = − c or b = − c . However, a, b and c are all positive integers, so no triplet exists.
Once you reach your second equation a 3 + b 3 + c 3 + 3 a 2 b + 3 a 2 c + 3 a b 2 + 3 b 2 c + 3 a c 2 + 3 b c 2 + 6 a b c = a 3 + b 3 + c 3 , it's obvious that the left-hand side must be greater than the right-hand side, so there are no solutions.
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I will use this identity─ ( a + b + c ) 3 = a 3 + b 3 + c 3 + 3 ( a + b ) ( b + c ) ( c + a )
As stated in the problem statement, ( a + b + c ) 3 = a 3 + b 3 + c 3 , so we can say─
0 = 3 ( a + b ) ( b + c ) ( c + a )
⟹ 0 = ( a + b ) ( b + c ) ( c + a )
⟹ a + b = 0 or b + c = 0 or c + a = 0
⟹ a = − b or b = − c or c = − a , which is a contradiction whenever a , b and c are positive integers.
So, False is the answer.