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Relating Length, Surface Area, and Volume
The Math IC tests not only whether you’ve memorized the
formulas for the different geometric solids, but also whether you
understand those formulas. The test gauges your understanding by
asking you to calculate the lengths, surface areas, and volumes
of various solids. The Math IC will ask you about the relationship
between these three properties. The Math IC includes two kinds of
questions covering these relationships.
Comparing Dimensions
The first way the Math IC will test your understanding
of the relationship among the basic measurements of geometric solids
is by giving you the length, surface area, or volume of different
solids and asking you to compare their dimensions. The math needed
to answer comparing-dimensions questions isn’t that hard. But in
order to do the math, you need to have a good grasp of the formulas
for each type of solid and be able to relate those formulas to one
another algebraically. For example,
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This question tells you that the surface area of a sphere
and the volume a cylinder are equal. A sphere’s surface area is 4π(rs)2,
where rs is
the radius of the sphere.
A cylinder’s volume is π(rc)2 
h,
where rc is
the radius of the cylinder, and h is
its height. Therefore,
h,
where rc is
the radius of the cylinder, and h is
its height. Therefore,2=pi(rx)2.gif)
The question asks for the ratio between the radii of the
sphere and the cylinder. This ratio is given by r
s/rc .
Now you can solve the equation 4πrs2 =
πrc2 
h for the ratio
rs/rc .2.gif)
h for the ratio Changing Measurements
The second way the Math IC will test your understanding
of the relationships among length, surface area, and volume is by
changing one of these measurements by a given factor, and then asking
how this change will influence the other measurements.
When the lengths of a solid in the question are increased
by a single constant factor, a simple rule can help you find the
answer:
- If a solid’s length is multiplied by a given factor, then the solid’s surface area is multiplied by the square of that factor, and its volume is multiplied by the cube of that factor.
Remember that this rule holds true only if all of
a solid’s dimensions increase in length by a given factor. So for
a cube or a sphere, the rule holds true when just a side or the
radius changes, but for a rectangular solid, cylinder, or other
solid, all of the length dimensions must change by the same factor.
If the dimensions of the object do not increase by a constant factor—for
instance, if the height of a cylinder doubles but the radius of
the base triples—you will have to go back to the equation for the
dimension you are trying to determine and calculate by hand.
Example 1
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If you understand the formula for the area of a square,
this question is simple. The formula for the area of a square is
A = s2,
where s is the length of a side. Replace s with
2s, and you see that the area of a
square quadruples when the length of its sides double: (2s)2 = 4s2.
Example 2
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The radius of the sphere is multiplied by a factor of 1⁄2 (or
divided by a factor of 2), and so its volume multiplies by the cube
of that factor: (1⁄2)3 = 1⁄8.
Therefore, the volume of the sphere is multiplied by a factor of 1⁄8 (divided
by 8), which is the same thing as decreasing by a factor of 8.
Example 3
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y 
z (these
are its length, width, and height), and a volume of 64. What is
the volume of a rectangular solid of dimensions 
z?If this rectangular solid had dimensions that were all
one-half as large as the dimensions of the solid whose volume is
64, then its volume would be (1⁄2)3 
64 = 1⁄8 
64 = 8. But dimension z is
not multiplied by 1⁄2 like x and y.
To answer a question like this one, you should use the volume formula
for rectangular solids: Volume = l 
w 
h.
It is given in the question that xyz =
64. So, x⁄2 
y⁄2 
z = 1⁄4 
xyz = 1⁄4 
64 = 16.
64 = 1⁄8 64 = 8. But dimension z is
not multiplied by 1⁄2 like x and y.
To answer a question like this one, you should use the volume formula
for rectangular solids: Volume = l w h.
It is given in the question that xyz =
64. So, x⁄2 y⁄2 z = 1⁄4 xyz = 1⁄4 64 = 16.|
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