'The equation of evolute of the ellipse.
Parameters
x : str, optional
Label for the x-axis. Default value is \'x\'.
y : str, optional
Label for the y-axis. Default value is \'y\'.
Returns
equation : sympy expression
Examples
>>> from sympy import Point, Ellipse
>>> e1 = Ellipse(Point(1, 0), 3, 2)
>>> e1.evolute()
2**(2/3)*y**(2/3) + (3*x - 3)**(2/3) - 5**(2/3)'

def evolute(self, x='x', y='y'):

    if (len(self.args) != 3):
        raise NotImplementedError('Evolute    of    arbitrary    Ellipse    is    not    supported.')
    x = _symbol(x)
    y = _symbol(y)
    t1 = ((self.hradius * (x - self.center.x)) ** Rational(2, 3))
    t2 = ((self.vradius * (y - self.center.y)) ** Rational(2, 3))
    return ((t1 + t2) - (((self.hradius ** 2) - (self.vradius ** 2)) ** Rational(2, 3)))
