SOLUTION: A solid cone has a height of 18cm and a radius of 6cm. From the apex, a shape to a height of 12 cm is cut off. Find the surface and volume of the remaining shape. Thank you :)

Question 1126070: A solid cone has a height of 18cm and a radius of 6cm. From the apex, a shape to a height of 12 cm is cut off. Find the surface and volume of the remaining shape. Thank you :)
Answer by greenestamps(13200) (Show Source): You can put this solution on YOUR website!

I answered this problem a few days ago.... My answer is copied below.

The shape that is cut off is a cone similar to the original cone. The height of the original cone is 18cm; the height of the cone that is cut off is 12cm. So the scale factor (ratio of similarity) between the smaller and larger cones is 2:3.

By a general principle regarding similar figures, the ratio of surface areas of the two cones will then be 2^2:3^2 = 4:9, and the ratio of volumes will be 2^3:3^3 = 8:27.

The volume of the original cone is

So the volume of the cone that is cut off is

Then the volume of the remaining frustum is

The surface area of the frustum is the area of the two bases, plus the portion of the lateral surface area of the original cone that is left after the small cone is cut off.

Using the ratio of similarity of the two cones, the radii of the two bases of the frustum are 6cm and 4cm; the area of those bases is

The lateral surface area of the original cone is

where l is the slant height. The slant height is found from the Pythagorean Theorem using the radius and height:

The lateral surface area of the original cones is then

The lateral surface area of the small cone that is cut off is 4/9 of that, leaving 5/9 of that as the lateral surface area of the frustum. So the lateral surface area of the frustum is

And finally, then, the surface area of the frustum is

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