A circle is centered at the origin ( 0 , 0 ) and has a radius of 4 units. What points on the circle have an x-coordinate of 1? (Your answer should be a list of points, such as "(1,1), (2,4)".) Which points on the circle have a y-coordinate of 3? (Your answer should be a list of points, such as "(1,1), (2,4)".) Write an equation for the graph of this circle.