Question 29·200 Super-Hard SAT Math Questions·Geometry and Trigonometry
Pyramid A is a right square pyramid with base side length 6 centimeters and height 4 centimeters. Pyramid B is similar to pyramid A.
The total surface area of pyramid B (including its base) is 480 square centimeters.
Which choice is the volume of pyramid B, in cubic centimeters?
When similar solids appear, decide which measurement gives you the scale factor: lengths scale by , surface areas by , and volumes by . Here, compute one complete surface area to find from the surface-area ratio, then compute one volume and multiply by —this avoids trying to build the larger pyramid directly.
Hints
Compute pyramid A’s surface area first
Find the slant height of a face using a right triangle with legs 4 and 3, then use it to compute the total surface area (including the base).
Relate the two surface areas
For similar solids, the ratio of surface areas equals the square of the linear scale factor .
Don’t forget volume uses a different power of the scale factor
Once you have , scale the volume of pyramid A by (not by or ).
Desmos Guide
Compute pyramid A’s total surface area
In Desmos, define s=6 and h=4. Then enter l=sqrt((s/2)^2+h^2) and SAa=s^2+2*s*l.
Compute the scale factor from surface areas
Enter SAb=480 and k=sqrt(SAb/SAa). This uses .
Compute and scale the volume
Enter Va=(1/3)*s^2*h and Vb=Va*k^3. Compare the expression shown for Vb to the answer choices and select the match.
Step-by-step Explanation
Find the total surface area of pyramid A
For a right square pyramid with base side length and height , the slant height of a face comes from a right triangle with legs and :
With and :
Total surface area (including base) is
So
Use surface area to get the linear scale factor
Since the pyramids are similar, if the linear scale factor from A to B is , then surface areas scale by :
So . (Use the positive root because lengths are positive.)
Scale the volume
First find the volume of pyramid A:
Volumes scale by , so
So the correct choice is .