55 points find the center and radius, then write the standard equation of each circle

The standard form form for a circle is , where h and k are the center's coordinates respectively, and r is the radius.  The first circle has a center at the origin, so the center's coordinates are (0, 0).  Counting from the center straight across or up or down to a point on the circle tells us that the radius is 5.  Our equation then is , or simplifying, .  The second circle has a center of (5, 4).  The radius is 3.  So the equation for that circle is .  The last circle has a center of (4, 3) and a radius of 5.  The equation for that circle is .  And there you go!