15. (a) The first part of equation

(S) x_{n+1} = x_{n}^{2} + a, y_{n+1} = 0

can be rewritten for a fixed point (x_{n+1} = x_{n})
as a quadratic equation x_{n}^{2} - x_{n} + 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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