Solution: cross point O as the perpendicular of AB, cross AB at point C and connect OA.

Let the radius of a circle be r.

According to the vertical diameter theorem, AC=BC= 1/2(AP+BP)=5.

Because AP=4, PC=AC-AP= 1

So OC= the square of OP under the root sign-the square of PC under the root sign =2× 6.

So r= the square of AC under the root sign+the square of OC under the root sign = 29.