15. (a) The first part of equation
(S) xn+1 = xn2 + a, yn+1 = 0
can be rewritten for a fixed point (xn+1 = xn) as a quadratic equation xn2 - xn + a = 0. Now apply the quadratic formula.
(b) For a = 1/4, x+ = x- = 1/2. For a = 0, x+ = 1 and x- = 0. For a = -3/4, x+ = 3/2 and x- = -1/2. For a = -2, x+ = 2 and x- = -1.
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