# The central angle and radius of a sector of a circular disc are 180° and 21 cm respectively. If the edges of the sector are joined together to make a hollow cone, then find the radius of the cone.

Given : Radius of circular disc = r = 21 cm

Central angle of sector = θ = 180°

To find : Radius of cone = R = ?

Formula : Area of sector =

CSA of cone = rl

Solution : When the edges of the sector are joined to form a cone, the radius of the sector becomes the slant height of the cone.

The area of the sector = CSA of the cone

21 × 21 = × R × 21

= 21R

R = = = 10.5 cm

The radius of the hollow cone formed by the sector of the circular disc is 10.5 cm.

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