Question 24·Hard·Area and Volume
Two right circular cones are similar. The height of the larger cone is times the height of the smaller cone. The volume of the smaller cone is cubic centimeters. What is the volume, in cubic centimeters, of the larger cone?
For SAT questions involving similar 3D figures (cones, spheres, prisms), first identify the linear scale factor between them from any length given (height, radius, edge). Then immediately convert that to a volume scale factor by cubing it (). Multiply the known volume by this factor instead of re-deriving formulas or finding each dimension separately. This saves time and helps avoid common errors like using or instead of .
Hints
Identify the scale factor
You know the cones are similar and the larger cone’s height is 4 times the smaller’s. What does that tell you about how every linear measurement (like the radius) changes from the smaller cone to the larger cone?
Connect similarity to volume
For similar 3D shapes, the ratio of volumes is not the same as the ratio of heights. How does volume change when every length is multiplied by a factor of ?
Apply the volume ratio
Once you know the volume scale factor from smaller to larger, multiply the smaller cone’s volume, , by this factor. Be careful to use , not just or .
Finish the arithmetic carefully
After you decide on the correct scale factor for volume, multiply that number by 27 to get the new coefficient in front of . Double-check the multiplication to avoid small mistakes.
Desmos Guide
Enter the expression for the larger volume
In Desmos, type 27 * pi * 4^3 to represent the smaller cone’s volume times the cube of the linear scale factor from smaller to larger.
Read the resulting value
Look at the numeric result Desmos shows for 27 * pi * 4^3. Identify the integer coefficient in front of ; that coefficient, with attached, gives the volume of the larger cone.
Step-by-step Explanation
Relate similarity to scale factor
For similar 3D shapes (like cones), all linear dimensions (height, radius, slant height) are multiplied by the same scale factor. Here, the height of the larger cone is 4 times the height of the smaller cone, so the linear scale factor from smaller to larger is .
Use how volume scales with linear dimensions
Volume is a 3D measure, so when all linear dimensions are multiplied by a factor , the volume is multiplied by .
Here, , so the volume scale factor from the smaller cone to the larger cone is
Apply the volume scale factor to the given volume
The smaller cone’s volume is given as cubic centimeters.
To get the larger cone’s volume, multiply the smaller volume by the volume scale factor :
Now compute to find the final volume.
Compute the product to find the larger cone’s volume
Multiply:
Alternatively:
So the larger cone’s volume is
Therefore, the correct answer is .