JAMB 1999 · UME · Q24

Solid mensuration

A frustum has top radius 3 cm, bottom radius 6 cm and vertical height 4 cm. Find its slant height $l$ .

Worked solution (try it first)

Draw the slant side as the hypotenuse of a right-angled triangle.
Its vertical side is the height, 4 cm.
Its horizontal side is the difference of the radii: $6 - 3 = 3$ cm.
Pythagoras: $l^2 = 4^2 + 3^2 = 25$ , so $l = 5$ cm, option A.

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