Circle a triangle whose base has the length of the circle's circumference and whose height equals the circle's radius, which comes to multiplied by the radius squared: Equivalently, denoting diameter by "d", that is, approximately 79% of the circumscribing square (whose side is of length "d"). The circle is the plane curve enclosing the maximum area for a given arc length. This relates the circle to a problem in the calculus of variations, namely the isoperimetric inequality. Equation of a circle<br> In an "x"–"y" Cartesian coordinate system, the circle with centre coordinates ("a", "b") and radius "r" is the set of
Circle Line Sightseeing Cruises location. In 1962 it bought the famous and venerable Hudson River Day Line, keeping it as a separate brand until 1985. In 1981, the Circle Line split into two companies—Circle Line Sightseeing Cruises and Circle Line Downtown. In 1988 the 42nd Street company bought World Yacht's operating upscale dining cruises from Chelsea Piers - currently World Yacht Dining Cruises. In 1998 the 42nd Street company also launched The Beast, a speedboat ride which takes tourists around the Statue of Liberty and goes 45 mph. In 2007, the United States National Park Service said it was going to terminate Circle Line