'The equation of the circle.
Parameters
x : str or Symbol, optional
Default value is \'x\'.
y : str or Symbol, optional
Default value is \'y\'.
Returns
equation : SymPy expression
Examples
>>> from sympy import Point, Circle
>>> c1 = Circle(Point(0, 0), 5)
>>> c1.equation()
x**2 + y**2 - 25'

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

    x = _symbol(x)
    y = _symbol(y)
    t1 = ((x - self.center.x) ** 2)
    t2 = ((y - self.center.y) ** 2)
    return ((t1 + t2) - (self.major ** 2))
