Equivalent Cube Like Equivalent Resistance

Algebra Level 2

TRUE or FALSE ?

There exist(s) a triplet/triplets of positive integers ( a , b , c ) (a,b,c) such that ( a + b + c ) 3 = a 3 + b 3 + c 3 . (a+b+c)^3=a^3+b^3+c^3.

Or Geometrically speaking,

There exist three cubes with integer side lengths a , b a, b and c c , respectively, such that the sum of their volumes equals the volume of another cube with side length ( a + b + c ) (a+b+c) .

False True

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2 solutions

I will use this identity─ ( a + b + c ) 3 = a 3 + b 3 + c 3 + 3 ( a + b ) ( b + c ) ( c + a ) (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 (a+b+c)^3 = a^3+b^3+c^3 , so we can say─

0 = 3 ( a + b ) ( b + c ) ( c + a ) 0 = 3(a+b)(b+c)(c+a)

0 = ( a + b ) ( b + c ) ( c + a ) \implies 0 = (a+b)(b+c)(c+a)

a + b = 0 or b + c = 0 or c + a = 0 \implies a+b = 0 \text{ or } b+c=0 \text{ or } c+a=0

a = b or b = c or c = a \implies a= -b \text{ or } b=-c \text{ or } c=-a , which is a contradiction whenever a , b a, b and c c are positive integers.

So, False \boxed{\text{False}} is the answer.

For 1st question, a=b=c=0 is possible.

Harsh Khasbage - 3 years, 5 months ago

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0 0 is not positive.

Muhammad Rasel Parvej - 3 years, 5 months ago
Joseph Newton
Dec 19, 2017

( 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 (a+b+c)^3=a^3+b^3+c^3\\ a^3+b^3+c^3+3a^2b+3a^2c+3ab^2+3b^2c+3ac^2+3bc^2+6abc=a^3+b^3+c^3\\ a^2b+a^2c+ab^2+b^2c+ac^2+bc^2+2abc=0\\ a^2c+2abc+b^2c+a^2b+ab^2+ac^2+bc^2=0\\ c(a^2+2ab+b^2)+ab(a+b)+c^2(a+b)=0\\ c(a+b)^2+(ab+c^2)(a+b)=0\\ (c(a+b)+ab+c^2)(a+b)=0\\ (ab+ac+bc+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=-b , a = c a=-c or b = c 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 , a^3+b^3+c^3+3a^2b+3a^2c+3ab^2+3b^2c+3ac^2+3bc^2+6abc=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.

Jon Haussmann - 3 years, 5 months ago

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