Let x = a + b 1 = b + c 1 = c + a 1 , where a , b , and c are distinct real numbers.
What is the sum of all the values of x ?
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Extremely simple!
x = a + b 1 → b = x − a 1 x = b + c 1 → c = x − b 1 = x − x − a 1 1 x = c + a 1 → a = x − c 1 = x − x − x − a 1 1 1 = x − x 2 − a x − 1 x − a 1 = x 3 − a x 2 − 2 x + a x 2 − a x − 1 a × ( x 3 − a x 2 − 2 x + a ) = x 2 − a x − 1 a x 3 − a 2 x 2 − 2 a x + a 2 = x 2 − a x − 1 a x 3 − a 2 x 2 − 2 a x + a 2 − x 2 + a x + 1 = 0 a x 3 − a 2 x 2 − a x + a 2 − x 2 + 1 = 0 a x 3 − a 2 x 2 − x 2 − a x + a 2 + 1 = 0 a x × x 2 − a 2 × x 2 − 1 × x 2 − a x + a 2 + 1 = 0 x 2 ( a x − a 2 − 1 ) − ( a x − a 2 − 1 ) = 0 ( x 2 − 1 ) ( a x − a 2 − 1 ) = 0 x 2 − 1 = 0 or a x − a 2 − 1 = 0 . If a x − a 2 − 1 = 0 , then a x − a 2 = 1 a ( x − a ) = 1 x − a = a 1 x = a + a 1 → a = b . Does not match the meaning of the title. So a x − a 2 − 1 = 0 → x 2 − 1 = 0 x 2 = 1 x = 1 or x = − 1 . The sum of all the values of x is 1 + ( − 1 ) = 0
For any solution x 1 = x ( a , b , c ) there is also a solution x 2 = x ( − a , − b , − c ) = − x ( a , b , c ) = − x 1 . The solutions come in pairs of opposite sign. If there is a finite number of solutions, ∑ x = 0 .
Note that a − b = b − 1 c − 1 ( b − c ) b − c = a − 1 c − 1 ( c − a ) c − a = a − 1 b − 1 ( a − b ) so that a − b = ( a b c ) − 2 ( a − b ) and hence a b c = ± 1 .
If a b c = 1 then a − b = a ( b − c ) and b − c = b ( c − a ) , so that 0 = ( a − b ) + ( b − c ) + ( c − a ) = ( a b + b + 1 ) ( c − a ) , and hence a b + b + 1 = 0 , so that x = a + b − 1 = − 1 . Note that a = 1 , b = − 2 1 , c = − 2 give an example of this case.
If a b c = − 1 then a − b = − a ( b − c ) and b − c = − b ( c − a ) , so that 0 = ( a − b ) + ( b − c ) + ( c − a ) = ( a b − b + 1 ) ( c − a ) , and hence a b − b + 1 = 0 , so that x = a + b − 1 = 1 . Note that a = 2 , b = − 1 , c = 2 1 give an example of this case.
Thus the answer is − 1 + 1 = 0 .
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