Find all values of x that satisfy \(x=1-x+x^2-x^3+x^4-x^5+ ...\)
x - 1 = x (x +x^3 + x^5 +.....) - 1(x + x^3 + x^5 + .....)
x - 1 = (x -1) (x + x^3 + x^5 + .......)
(x -1) / ( x -1) = ( x + x^3 + x^5 + ......)
1 = ( x + x^3 + x^5 + .......)
-1 = -x - x^3 - x^5 + .........
So
x = 1 - 1 + x^2 +x^4 + x^6+ ........
x = x^2 + x^4 + x^6 + .......
Infinite sum
x^2 / ( 1 - x^2) = x
x^2 = x - x^3
x^3 + x^2 - x = 0
x ( x^2 + x - 1) = 0
x = 0 reject
x^2 + x - 1 = 0
x^2 + x = 1
x^2 + x + 1/4 = 1 + 1/4
(x + 1/2)^2 = 5/4
x + 1/2 = sqrt (5) / 2 or x + 1/2 = -sqrt (5) / 2
x = [ sqrt (5) - 1 ] 2 or x = [ -sqrt (5) -1 ] / 2
x = "phi" or x = "-Phi "