Help figuring out how to tell if an object is within a dynamically made circle

Hello Everyone,

I am currently in the middle of working on a project but sadly I am having a bit of trouble getting a few things figured out.

Essentially what the project consists of is I have random balls appearing on the screen and moving around and I need to make it so that the user is able to draw a circle around the ball (may be more than one) and have that balls dissapear and count the number of balls circled.

This project is being done in flash builder 4.5 and is being made for the android platform as part of a senior project.

At this point this is where I am.. I have been able to randomly create the balls and give them a random movement. I have also been able to figure out how to allow the user to draw on the screen.

What I am trying to figure out is how to make it so that if the user draws a circle around the balls make it so that I know these balls have been circled and make them dissapear and have the number of balls inside the area calculated.

I have thought about making the entire background a grid and make it so that I can track somehow the line made by the user and then track the coordinate of the balls then if it is within that area it means that the ball has been circled but I would like something more efficient than that.

I apologize if it is unclear for what I am looking for but a simple description is I am making a simple game where you are able to circle a moving item on screen, then once it is circled the item disappearsand you gain a point.

Any help would be greatly appreciated.