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Solids Produced by Rotating Polygons
Another type of Math IIC question that you may come across
involves a solid produced by the rotation of a polygon. The best
way to explain this type of problem is with an example:
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about the axis AB?
When this triangle is rotated about AB,
a cone is formed. To solve the problem, the first thing you should
do is sketch the cone that the triangle will form.
The question asks you to figure out the surface area of
the cone. The formula for surface area is πr2 + πrl,
which means you need to know the lateral height of the cone and
the radius of the circle. If you’ve drawn your cube correctly, you
should see that the lateral height is equal to the hypotenuse of
the triangle. The radius of the circle is equal to side BC of
the triangle. You can easily calculate the length of BC since
the triangle is a 30-60-90 triangle. If the hypotenuse is 2, then BC,
being the side opposite the 30º angle, must be 1. Now plug both
values of l and r into the surface
area formula and then simplify:
Common Rotations
You don’t need to learn any new techniques or formulas
for problems dealing with rotating figures. You just have to be
able visualize the described rotation and be aware of which parts
of the polygons become which parts of the geometric solid. Below
is a summary of which polygons, when rotated a specific way, produce
which solids.
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| A rectangle rotated about its edge produces a cylinder. | A semicircle rotated about its diameter produces a sphere. | A right triangle rotated about one of its legs produces a cone. |
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| A rectangle rotated about a central axis (which must contain the midpoint of both of the sides that it intersects) produces a cylinder. | A circle rotated about its diameter produces a sphere. | An isosceles triangle rotated about its axis of symmetry (the altitude from the vertex of the non-congruent angle) produces a cone. |
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