Convert the cartesian equation x2 +y2=16 to a polar equation

X^2+y^2 = 16
can be written as
(x-0)^2+(y-0)^2 = 4^2

We see that the second equation is in the form
(x-h)^2 + (y-k)^2 = r^2

where
(h,k) = (0,0) is the center
r = 4 is the radius

The polar form of the equation is simply r = 4. Why is this? Because the radius is fixed to be 4 no matter what happens with theta. As theta goes from 0 to 360, the points generated all form a circle centered at (0,0) with radius 4. 

r = 4