Can you answer this hard question

See the problem at:

http://www.flickr.com/photos/mrdmbob/

Thanks for your help!

This will be hard to explain without pictures, but I'll give it a shot.

First find the radius (r):

(8+10)/2 = r
r = 18/2
r = 9

Then find the length of OP.
OP = r - 8
OP = 9 - 8
OP = 1

Let's call the point where the top of the dotted line intersects the curve of the circle 'Q'.
So now you have a triangle OPQ, and you know the length of OP = 1 and OQ = r = 9.
Now comes Pythagoras!
OP^2 + QP^2 = OQ^2
9^2 - 1^2 = QP^2
81 - 1 = QP^2
80 = QP^2
QP = sqrt(80)
QP = 8.9
So the length of the dotted line is approximate 8.9 (units).