on number 13 Im confused. I thought a-b would be pythagorus so sqrt (2r^2) .... It gives sqrt(2r)?
I'm assuming you're referring to exercise 13(a)i, the straight-line distance between A and B.
Since the triangle (inside the quarter-circle) is isosceles right, with equal-length legs "r", the hypotenuse is, as you suggest, given by:
. . . . .
I would guess that is a typo.