AB and CD are respectively arcs of two concentric circles of radii 21 cm and 7 cm and centre O (see Fig. 12.32). If ∠ AOB = 30°, find the area of the shaded region
Area of the shaded region = Area of sector OAEB - Area of sector OCFD
= 
* 
* (21)2 - 
* 
* (7)2
= 
* 
[(21)2 – (7)2]
= 
* 
[(21 – 7) (21 + 7)]
= 
= 
cm2
18