11. Let A1, A2, A3,..., As be the vertices of a regular octagon that lie on a circle of radius 2. Let P be a point on the circle and let PA, denote the distance between the points P and A, for i = 1, 2, , 8. If P varies over the circle, then the maximum value of the product PA1-PA2....PA8, is a 3 b 12. Let R c 2 da,b,c,d = {0,3,5,7,11,13,17,19} Then the number of invertible matrices in R is 0 50 13. Let C be the circle of radius 1 with center at the origin. Let C be the circle of radius r with center at the point A = (4,1), where 1