import sympy as sm

z, a, x_0, x_d = sm.symbols("z, a, x_0, x_d")
X_z = ((1 + a) / 2 * z / (z - 1) * x_d + z * x_0) / ((2 * z - (1 - a)) / 2)
sm.pprint(X_z)
sm.pprint(X_z.apart(z))