Question

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.

Answer 1

radius pf outer circle is 21cm
area of sector of AOB =thete÷ 360 πrsq.
30÷360×22÷7×21×21=1286/12
area of sector of inner circle= theta/360πrsq
= 154/12
area of shaded region=area of sector AOB -area of another sector
1286/12-154-12=1132/6

Answer 2

Sector of a circle:
The region enclosed by two radio & the corresponding arc of a circle is called the sector of a circle.

The sector contain minor sector and major sector.

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Solution:

[Fig is in the attachment]

Given:

∠AOB = 30°
Smaller Radius OC (r) = 7 cm
Bigger Radius OB(R)= 21 cm

Angle made by sectors of both concentric circles (thetha) =30°

Area of the larger sector AOB = (30°/360°) × π R² cm²

= 1/12 × 22/7 ×( 21)² cm²

= 693/6 cm²

Area of the smaller sector COD= (30°/360°) × π r² cm²

= 1/12 × 22/7 × 72 cm²

= 77/6 cm²

Area of shaded region= area of sector AOB - area of sector COD

Area of the shaded region =693/6 – 77/6 cm²

= 616/6 cm² = 308/3 cm²

Hence, the area of shaded region is 308/3 cm²

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Hope this will help you.....