src/Maths/Geometry/Circle.php
public function isValidPoint(PointInterface $a)
    {
        $o = $this->getPointO();
        return (bool) (
            (pow(($a->getAbscissa() - $o->getAbscissa()), 2) + pow(($a->getOrdinate() - $o->getOrdinate()), 2)) == pow($this->getRadius(), 2)
        );
    }
public function isValidPoint(PointInterface $a)
    {
        $o = $this->getPointO();
        return (bool) (
            (pow(($a->getAbscissa() - $o->getAbscissa()), 2) + pow(($a->getOrdinate() - $o->getOrdinate()), 2)) == pow($this->getRadius(), 2)
        );
    }
(x - Ox)^2 + (y - Oy)^2 = r^2

@param   \Maths\PointInterface   $a
@return  bool