i have been given the assignment to form the differential equation of circle...

if anyone can please help me...i'll be very thankfu to him...

first..i have to find out the differential equation of a circle with radius r...whose equaton is (x-a)^2 + (y-b)^2 = r^2..

and secondly..i have to find out the differential equation of a circle that passes through origin..

i dont even know whatst the equation of circle that passes through origin...please help...