NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.7 – Part 4

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Surface area of cone= πrl

Surface Area of Cone

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Q-8 If the triangle BEC in the Question 7 above is revolved about the side 5 cm, and then fined the volume of the solid so obtained. Find also the ratio of the volumes of the two solids obtained in Questions 7 and 8.If the triangle is revolved about the side of 5 cm then the 12 cm side becomes the radius of the cylinder and the 13 cm side remains the slant height.


Triangle BEC Rotated to Form a Cone

Given, a right circular cone with

  • Radius

  • Height

Therefore, volume

Now, from previous problem, the right circular cone base is and height

Therefore, volume

Ratio of the volumes = or

Q-9 A heap of wheat is in the form of a cone whose diameter is and height is Find its volume. The heap is to be covered by canvas to protect it from rain. Find the area of the canvas required.


  • Diameter of the base of the cone

  • Radius

  • Height of the cone

Volume of the heap is

Also to calculate the slant height,

Area of canvas = Curved surface area of the cone

  • That is,

  • (approx.)