A right triangle whose sides are 15 cm and 20 cm is made to revolve about its hypotenuse. Find the volume and surface area of the double cone so formed.

(Ans : 3768cu.cm,1318.8  Sq.cm)

Ans:    BC = √15² + √20²

= 25 cm

Apply Py. Th to right ΔOAB & OAC to get OB = 9cm       OA = 12cm

Vol of double cone     = vol of CAA¹ + vol of BAA¹

= 1/3 π × 12²  × 16 + 1/3 π × 12² × 9

= 3768 cm³

SA of double cone      = CSA of CAA¹ + CSA of BAA¹

= π ×  12 × 20 +π × 12 × 15

= 1318.8 cm³