Find the surface area of the circular paraboloid$\displaystyle z = x^2+ y^2$between

the xy-plane and the plane z = 4.

The limits have suddeny become for double integration

2pi 0

and 3 and 0

dr dtheta

I know

r = (x^2 + y^2)^0.5

x = r cos theta

y = r sin theta

and theta = inverse tan (x/y)

How do i get these limits? Thanks.