Answer:
Prob.3
Denote r: radius of circle O
Formula for calculating AOB sector:
Area = pi*r^2*angle/360
Area = pi*r^2*45/360 = 18 pi
=> r^2 = 18*360/45 = 144
=> r = 12
For prob.4, give it a try, follow the formula I gave. Good luck !
(answer : r = 6)
