def _minpoly_cos(ex, x):
    from sympy import sqrt
    c, a = ex.args[0].as_coeff_Mul()
    if a is pi:
        if c.is_rational:
            if c.p == 1:
                if c.q == 7:
                    return 8*x**3 - 4*x**2 - 4*x + 1
                if c.q == 9:
                    return 8*x**3 - 6*x + 1
            elif c.p == 2:
                q = sympify(c.q)
                if q.is_prime:
                    s = _minpoly_sin(ex, x)
                    return _mexpand(s.subs({x:sqrt((1 - x)/2)}))

            # for a = pi*p/q, cos(q*a) =T_q(cos(a)) = (-1)**p
            n = int(c.q)
            a = dup_chebyshevt(n, ZZ)
            a = [x**(n - i)*a[i] for i in range(n + 1)]
            r = Add(*a) - (-1)**c.p
            _, factors = factor_list(r)
            res = _choose_factor(factors, x, ex)
            return res

    raise NotAlgebraic("%s doesn't seem to be an algebraic element" % ex)
Returns the minimal polynomial of ``cos(ex)``
dataset/ETHPy150Open sympy/sympy/sympy/polys/numberfields.py/_minpoly_cos