The frustum of a right circular cone has the diameter of a base 10 cm, of top…

2019

The frustum of a right circular cone has the diameter of a base 10 cm, of top 6 cm and a height of 5 cm. Find the slant height of the frustum:

  1. A.

    √29

  2. B.

    4√3

  3. C.

    3√3

  4. D.

    √13

Attempted by 10 students.

Show answer & explanation

Correct answer: A

Solution:

Step-by-step reasoning to find the slant height of the frustum.

  • Find the radii from the given diameters: radius of the base = 10/2 = 5 cm; radius of the top = 6/2 = 3 cm.

  • The horizontal leg of the right triangle (along the sloping side) is the difference of the radii: 5 − 3 = 2 cm.

  • The vertical leg is the height of the frustum: 5 cm.

  • Apply the Pythagorean theorem to these two legs to get the slant height l:

l = √[(difference of radii)^2 + (height)^2] = √(2^2 + 5^2) = √(4 + 25) = √29.

Therefore the slant height of the frustum is √29 cm (approximately 5.385 cm).

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