What is the sum of all the values of x?

Algebra Level 4

Let x = a + 1 b = b + 1 c = c + 1 a x=a+\dfrac{1}{b}=b+\dfrac{1}{c}=c+\dfrac{1}{a} , where a a , b b , and c c are distinct real numbers.

What is the sum of all the values of x x ?


The answer is 0.

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4 solutions

Andrew Penner
Jan 25, 2021
  1. Let x^3=(a+1/a)(b+1/b)(c+1/c)=abc+1/(abc)+a+1/a+b+1/b+c+1/c
  2. x^3=abc+1/(abc)+3x
  3. Replace abc with N
  4. x^3-0*x^2-3x-N-1/N=0
  5. The sum of the roots of a cubic, by Vieta’s formula=-b/a=0/1=0

Extremely simple!

Raymond Fang - 4 months, 2 weeks ago
Raymond Fang
Jan 24, 2021

x = a + 1 b b = 1 x a x = b + 1 c c = 1 x b = 1 x 1 x a x = c + 1 a a = 1 x c = 1 x 1 x 1 x a = 1 x x a x 2 a x 1 = x 2 a x 1 x 3 a x 2 2 x + a x=a+ \frac{1}{b} \rightarrow b= \frac {1}{x-a} \newline x=b+ \frac{1}{c} \rightarrow c= \frac{1}{x-b} = \frac{1}{x-\frac{1}{x-a}} \newline x=c+\frac{1}{a} \rightarrow a=\frac{1}{x-c} = \LARGE{\frac{1}{x-\frac{1}{x-\frac{1}{x-a}}}} \newline =\LARGE{\frac{1}{x-\frac{x-a}{x^2-ax-1}}} \newline =\LARGE{\frac{x^2-ax-1}{x^3-ax^2-2x+a}} \newline 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 a \times (x^3-ax^2-2x+a) = x^2-ax-1 \newline ax^3-a^2x^2-2ax+a^2=x^2-ax-1 \newline ax^3-a^2x^2- 2ax+a^2-x^2+ ax+1=0 \newline ax^3-a^2x^2- ax+a^2-x^2+1=0 \newline ax^3-a^2x^2-x^2-ax+a^2+1=0 \newline ax \times x^2 - a^2 \times x^2 - 1 \times x^2- ax + a^2 + 1 =0 \newline x^2(ax-a^2-1)-(ax-a^2-1)=0 \newline (x^2-1)(ax-a^2-1)=0 \newline x^2-1=0 or a x a 2 1 = 0. ax-a^2-1=0. \newline If a x a 2 1 = 0 , ax-a^2-1=0, then a x a 2 = 1 a ( x a ) = 1 x a = 1 a x = a + 1 a a = b ax-a^2=1 \newline a(x-a)=1 \newline x-a=\frac{1}{a} \newline x=a+\frac{1}{a} \rightarrow 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 ax-a^2-1 \neq 0 \rightarrow x^2-1=0 \newline x^2=1 \newline x=1 or x = 1 x=-1 . The sum of all the values of x is 1 + ( 1 ) = 0 1+(-1)=\boxed{0}

K T
Jan 25, 2021

For any solution x 1 = x ( a , b , c ) x_1=x(a , b, c) there is also a solution x 2 = x ( a , b , c ) = x ( a , b , c ) = x 1 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 \sum x =0 .

Mark Hennings
Jan 25, 2021

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 ) a-b = b^{-1}c^{-1}(b-c) \hspace{1cm} b-c = a^{-1}c^{-1}(c-a) \hspace{1cm} c-a = a^{-1}b^{-1}(a-b) so that a b = ( a b c ) 2 ( a b ) a-b = (abc)^{-2}(a-b) and hence a b c = ± 1 abc = \pm1 .

  • If a b c = 1 abc=1 then a b = a ( b c ) a-b = a(b-c) and b c = b ( c a ) b-c = b(c-a) , so that 0 = ( a b ) + ( b c ) + ( c a ) = ( a b + b + 1 ) ( c a ) 0 = (a-b)+(b-c)+(c-a) = (ab + b + 1)(c-a) , and hence a b + b + 1 = 0 ab + b + 1 = 0 , so that x = a + b 1 = 1 x = a + b^{-1} = -1 . Note that a = 1 a=1 , b = 1 2 b=-\tfrac12 , c = 2 c=-2 give an example of this case.

  • If a b c = 1 abc=-1 then a b = a ( b c ) a-b=-a(b-c) and b c = b ( c a ) b-c=-b(c-a) , so that 0 = ( a b ) + ( b c ) + ( c a ) = ( a b b + 1 ) ( c a ) 0 = (a-b)+(b-c)+(c-a) = (ab - b + 1)(c-a) , and hence a b b + 1 = 0 ab - b + 1 = 0 , so that x = a + b 1 = 1 x = a + b^{-1} = 1 . Note that a = 2 a=2 , b = 1 b=-1 , c = 1 2 c=\tfrac12 give an example of this case.

Thus the answer is 1 + 1 = 0 -1 + 1 = \boxed{0} .

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