Squares In Between

{ a + b + c = 11 a 3 + b 3 + c 3 = 785 \begin{cases} a + b + c = 11\\ a^3 + b^3 + c^3 = 785 \end{cases}

If a , b , c a, b, c are the integers, whose sum of any two variables is larger than 1, that satisfy the system above, what is the value of a 2 + b 2 + c 2 a^2 + b^2 + c^2 ?


The answer is 101.

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1 solution

( a + b + c ) 3 = a 3 + b 3 + c 3 + ( 3 a b 2 + 3 a c 2 + 3 b a 2 + 3 b c 2 + 3 c a 2 + 3 c b 2 + 6 a b c ) (a + b + c)^3 = a^3 + b^3 + c^3 + (3ab^2 + 3ac^2 + 3ba^2 + 3bc^2 + 3ca^2 + 3cb^2 +6abc)

( a + b + c ) 3 ( a 3 + b 3 + c 3 ) = 3 ( a b 2 + a c 2 + b a 2 + b c 2 + c a 2 + c b 2 + 2 a b c ) = 3 ( a + b ) ( b + c ) ( c + a ) (a + b + c)^3 - (a^3 + b^3 + c^3) = 3(ab^2 + ac^2 + ba^2 + bc^2 + ca^2 + cb^2 + 2abc) = 3(a + b)(b + c)(c + a)

From the system above, ( a + b + c ) 3 ( a 3 + b 3 + c 3 ) = 1 1 3 785 = 546 = 3 ( a + b ) ( b + c ) ( c + a ) (a + b + c)^3 - (a^3 + b^3 + c^3) = 11^3 - 785 = 546 = 3(a + b)(b + c)(c + a)

Hence, ( a + b ) ( b + c ) ( c + a ) = 182 = 2 × 7 × 13 (a + b)(b + c)(c + a) = 182 = 2\times 7\times 13 .

Since a , b , c a, b, c are integers, each sum of the two variables will equal to each of the three primes presented. Covering the cyclic solutions, we can set up the new system of equations as followed:

a + b = 2 a + b = 2 b + c = 7 b + c = 7 c + a = 13 c + a = 13

Then c a = 5 c -a = 5 , and 2 c = 18 ; c = 9 2c = 18; c = \boxed{9} .

Hence, a = 4 a = \boxed{4} , and b = 2 b = \boxed{-2} .

Thus, a 2 + b 2 + c 2 = 4 2 + ( 2 ) 2 + 9 2 = 101 a^2 + b^2 + c^2 = 4^2 + (-2)^2 + 9^2 = \boxed{101} .

Moderator note:

When manipulating equations, we have to be careful that we didn't introduce extraneous roots.

As such, after calculating the values of a , b , c a, b, c , we still have to ensure that they satisfy the original system of equations.

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